Metallic mesh-based gas diffusion electrodes for utilization of sparingly soluble gases in electrochemical reactions with nonaqueous electrolytes

ABSTRACT

A system and method for supplying a gas to an electrochemical system is described.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Application No.63/000,458, filed Mar. 26, 2020, which is incorporated by reference inits entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant No.CBET1944007 awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to gas diffusion electrodes.

BACKGROUND

Electrochemical transformations in nonaqueous solvents are important forsynthetic and energy storage applications. Use of nonpolar gaseousreactants such as nitrogen and hydrogen in nonaqueous solvents can belimited by their low solubility and slow transport. Conventional gasdiffusion electrodes can improve transport of gaseous species in aqueouselectrolytes by facilitating efficient gas-liquid contacting in thevicinity of the electrode. Conventional gas diffusion electrodes cannotimprove the transport in many nonaqueous electrolytes, however, as thehydrophobic interactions necessary for creating gas-liquid contactingare not present in nonaqueous electrolytes. This can lead to flooding ofthe electrode and low rates for gas utilization when using nonaqueouselectrolytes.

SUMMARY

In one aspect, an electrochemical system can include a housing includinga chamber, an electrode within the housing, and a gas permeable metal ona surface of the electrode in contact with the chamber.

In another aspect, a method of supplying a gas to an electrochemicalsystem can include contacting a gas with a gas permeable metal on asurface of an electrode in a chamber of a housing. The gas can be aprecursor that is converted to a reactive gas by the electrode.

In another aspect, a method of oxidizing or reducing a gas can includecontacting a gas with a gas permeable metal on a surface of anelectrode. The gas can be a sparingly soluble gas. The sparingly solublegas can be nitrogen or hydrogen. In certain circumstances, the ammoniacan be produced at a Faradaic yield of at least 30% or at least 40%. Incertain circumstances, the method can include supplying a pressure ofthe gas in the chamber to create a pressure differential at theelectrode. The method can allow for the use of a sparingly soluble gasas a reagent in chemical reactions.

In another aspect, an electrochemical system can include a firstelectrode including a housing including a chamber, an electrode withinthe housing, and a gas permeable metal on a surface of the electrode incontact with the chamber, and a second electrode including a gas inletto a housing including a gas permeable metal on a surface of anelectrode and a first outlet to release a product from the system.

In certain circumstances, the system can include a gas inlet to thehousing.

In certain circumstances, the system can include a first outlet of thehousing to release a product from the housing.

In certain circumstances, the gas permeable metal can include a porousmetal or a metal mesh system. In certain circumstances, each gaspermeable metal can include a metal mesh system. In certaincircumstances, the metal mesh can include 100, 200, 300, 400 or 500fibers per inch. In other circumstances, the metal mesh can beasymmetric and include 100, 200, 300, 400 or 500 fibers per inch in onedirection and 500, 1000, 1500, or 2000 fibers per inch in a seconddirection.

In certain circumstances, the gas permeable metal can include openingsof between 1 and 200 micrometers, preferably between 2 and 100micrometers.

In certain circumstances, each gas permeable metal can include openingsof between 1 and 200 micrometers, preferably between 2 and 100micrometers.

In certain circumstances, the gas permeable metal can include metalfibers or a porous metal.

In certain circumstances, at least one gas permeable metal can includemetal fibers or a porous metal.

In certain circumstances, the gas permeable metal can include stainlesssteel, steel, nickel, iron, copper, silver, gold, or platinum.

In certain circumstances, the gas permeable metal can include a catalyston a surface of the gas permeable metal. For example, the catalyst caninclude a surface treated with catalytic nanoparticles or catalyticnanoparticles deposited on the surface. In certain circumstances, thegas permeable metal can include a catalyst, for example, a catalyticmetal, metal oxide, metal sulfide, or metal phosphide.

In certain circumstances, the gas permeable metal can be exposed to apressure gradient. In certain circumstances, at least one gas permeablemetal can be exposed to a pressure gradient. In certain circumstances,the method can include supplying a pressure of the gas in the chamber tocreate a pressure differential in the housing.

In certain circumstances, the method can include applying a voltage tothe electrode.

Other aspects, embodiments, and features will be apparent from thefollowing description, the drawings, and the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1D depict a comparison of gas-liquid interfaces in variouselectrode setups. FIG. 1A shows gas diffusion through bulk aqueouselectrolyte to a flooded electrode. Note that a nonaqueous electrolyteperforms analogously in this system. FIG. 1B shows a traditionalhydrophobic carbon fiber-based gas diffusion electrode with an aqueouselectrolyte. FIG. 1C shows a traditional carbon fiber-based gasdiffusion electrode in the absence of hydrophobic repulsion when anonaqueous electrolyte is used. FIG. 1D shows a metallic support-basedgas diffusion electrode for use in nonaqueous electrolytes.

FIG. 2 depicts a graph showing hydrogen oxidation at high rates onplatinum-coated stainless steel cloths occurs in THF-based electrolyteat the anode, while nitrogen reduction on lithium-coated stainless steelcloths occurs at the cathode.

FIG. 3 depicts a graph comparing performance enabled by thearchitectural advances presented here (this work; stars) relative towork done in the literature (all other points). The system describedherein achieves record-high rates at relatively high Faradaicefficiencies.

FIG. 4 depicts a schematic showing a box half filled with liquid,including the relative energies of the liquid and gas phases in the box.

FIG. 5 depicts a schematic showing a box half filled with liquid havinga wall, including the relative energies of the liquid and gas phases inthe box.

FIG. 6 depicts a schematic showing a box half filled with liquid havinga wall and a metastable configuration, including the relative energiesof the liquid and gas phases in the box.

FIG. 7 depicts a schematic showing a box half filled with liquid havinghorizontal electrodes at a gas-liquid interface, including the relativeenergies of the liquid and gas phases in the box.

FIG. 8 depicts a schematic showing a box half filled with liquid havinga hydrophobic GDE or a non-hydrophobic GDE, including the relativeenergies of the liquid and gas phases in the box.

FIG. 9 depicts a schematic showing a box half filled with liquid havinga gas pressure gradient, including the relative energies of the liquidand gas phases in the box.

FIGS. 10A-10C depict kinetic and transport considerations forlithium-mediated nitrogen reduction. FIG. 10A shows reactions present ina lithium-mediated catalytic cycle for nitrogen reduction. (FIG. 10B,FIG. 10C) Diffusion limitations observed in electrochemical reactionsinvolving sparingly soluble gases (FIG. 10B) hydrogen and (FIG. 10C)nitrogen in a 1 M LiBF₄, 0.11 M ethanol in tetrahydrofuran electrolyteat flooded platinum and steel electrodes, respectively. The data in FIG.10B is collected by performing a linear sweep voltammogram at 5 mV s⁻¹.The dashed line in FIG. 10B helps to guide the eye. The solid line inFIG. 10C is a fit of the data to a kinetic-transport model for ammoniaproduction. Error bars in FIG. 10C are one standard deviation ofmultiple replicates (n≥2).

FIGS. 11A-11D depict structure of a gas diffusion electrode (GDE). FIG.11A shows a hydrophobic GDE with an aqueous electrolyte, wherewell-defined gas-liquid contacting exists. FIG. 11B shows a hydrophobicGDE with a nonaqueous electrolyte, where considerable wetting of thecarbon fibers occurs, effectively flooding the catalyst. FIG. 11C show acatalyst-coated steel cloth. A lack of significant capillary action andthe presence of a non-zero pressure gradient across the cloth preventcomplete catalyst flooding. FIG. 11D shows proton donor cycling in acell with a proton-producing anode.

FIGS. 12A-12D depict efficiency of the steel cloth-based GDEs for thehydrogen oxidation reaction (HOR) and the nitrogen reduction reaction(NRR). FIG. 12A shows a comparison of HOR Faradaic efficiency (FE) ofPt-coated steel cloths (Pt/SSC) and Pt-loaded carbon papers (Pt/C) atdifferent pressure gradients across the GDEs. FIG. 12B shows the effectof pressure gradient across a Pt/SCC on HOR FE at 25 mA cm⁻² appliedcurrent density. FIG. 12C shows the production rate of ammonia as afunction of applied current density on steel cloth cathodes at apressure gradient of 1 kPa across the steel cloth. Solution phaseammonia is found in the electrolyte while gas phase ammonia in the acidtrap after the cell. FIG. 12D shows the effect of pressure gradientacross a steel cloth cathode on FE toward NH₃ at 15 mA cm⁻² appliedcurrent density. Vertical error bars in FIGS. 12A and 12B represent acombination of uncertainty in HOR quantification and standard deviationbetween experiments (n≥2), while in FIGS. 12C and 12D they represent onestandard deviation between multiple replicates of the same experiments(n≥2). Horizontal error bars in FIGS. 12B and 12D represent the range ofpressure gradient values required for gas flow through the SSCs. Rawdata can be found in FIGS. 37A-37D and Tables 2-5. The dashed lines inFIGS. 12B and 12D represent the onset of gas breakthrough in the SSC,which is the Laplace pressure. In all experiments, 7.2 C of charge werepassed to measure either HOR or NRR FE.

FIGS. 13A-13D depict coupling of electrodes for a sustainable overallreaction. FIG. 13A shows a comparison of continuous ammonia productionmetrics at ambient conditions between this work and reported highestrates in nonaqueous electrolytes in the literature. See, for example,Lazouski, N., Schiffer, Z. J., Williams, K. & Manthiram, K.Understanding Continuous Lithium-Mediated Electrochemical NitrogenReduction. Joule 3, 1127-1139 (2019); Andersen, S. Z. et al. A rigorouselectrochemical ammonia synthesis protocol with quantitative isotopemeasurements. Nature 570, 504-508 (2019); and Tsuneto, A., Kudo, A. &Sakata, T. Lithium-mediated electrochemical reduction of high pressureN₂ to NH₃ . J. Electroanal. Chem. 367, 183-188 (1994), each of which isincorporated by reference in its entirety. FIG. 13B shows changes inFaradaic efficiency toward ammonia with different anode chemistries inexperiments where 7.2 C of charge were passed at an applied currentdensity of 20 mA cm⁻², with 10 standard cubic centimeters per minute ofgas flowing past the electrode, across which the pressure gradient is 1kPa. Error bars represent the standard deviation of multiple replicatesof the same experiment (n≥2). The insets show the anolyte afterlonger-term continuous operation at 20 mA cm⁻² for one hour atrespective anodes. The dark solution (Pt foil) contains poorly-definedtetrahydrofuran (THF) oxidation products, while the clear solution(Pt/SCC) shows few signs of THF oxidation. FIG. 13C shows a schematic ofan electrochemical Haber-Bosch (eHB) reactor coupled to awater-splitting reactor. FIG. 13D shows a photograph depicting a modelof an eHB reactor coupled to a water electrolyzer, with the reactorshighlighted.

FIGS. 14A-14D depict scanning electron microscopy (SEM) images stainlesssteel cloth electrodes. A Zeiss-Merlin HR-SEM with an HE-SE2 detectorwas used to collect images. FIG. 14A shows low magnification image ofbare stainless steel cloths (SSCs) (FIGS. 19A-19F). FIG. 14B showsmedium magnification images of nickel-coated SSCs (FIGS. 20A-20F). FIG.14C shows high magnification images of smoothly platinum-coated SSCs(FIGS. 21A-21F). FIG. 14D shows high magnification images of roughlyplatinum-coated SSCs (FIGS. 22A-22F). Note that the HOR FE does notdiffer between smoothly and roughly platinum-coated SSCs.

FIGS. 15A-15C depict control experiments confirming nitrogen reductionto ammonia. FIG. 15A shows a comparison between the Faradaic efficiencytoward ammonia when various gases are fed to the cell. When using N₂with different isotopic compositions, the ammonia yields are practicallyidentical, which is a sign that N₂ reduction is responsible for ammoniaformation. See, for example, Andersen, S. Z. et al. A rigorouselectrochemical ammonia synthesis protocol with quantitative isotopemeasurements. Nature 570, 504-508 (2019), which is incorporated byreference in its entirety. There is little to no ammonia formed when Aris used as the feed gas and in the absence of current. Vertical errorbars represent the uncertainty in Faradaic efficiency quantification ofa single experiment. FIG. 15B shows the amount of ammonia quantified inthe base and acid traps used to clean the inlet gas, and theconcentration of ammonia in a post-cell acid trap for comparison. FIG.15C shows unscaled NMR spectra of electrolyte and acid trap solutions.When ¹⁴N₂ is used as the feed gas, only a triplet from ¹⁴NH₄ ⁺ isdetected in both the trap and solution, while both ¹⁵NH₄ ⁺ and ¹⁴NH₄ ⁺are detected when ¹⁵N₂ is fed. ˜92% of the NH₄ ⁺ is ¹⁵NH₄, whichsuggests some ¹⁴N₂ contamination in the experiment, as the nominalisotopic content of the ¹⁵N₂ is 98%. The peaks shift slightly due todifferences in solvent composition (THF-water mixtures). The peak at˜6.87 is from butylated hydroxytoluene (BHT) found in the THF. The 25 mAexperiments were performed by using a 3-compartment cell with a platinumfoil anode, while the 20 mA experiments used a cell with no separatorbetween electrolyte compartments and a Pt/SSC anode.

FIG. 16 depicts a linear sweep voltammograms of hydrogen oxidation onPlatinum-coated carbon fiber gas diffusion electrodes in 0.5 M H₂SO₄ inwater. The sweep rate used is 5 mV s⁻¹. Hydrogen oxidation occurs athigh rates on hydrophobic platinum-loaded carbon paper (Pt/C) both whengas flows past the GDE (no pressure gradient applied) and when it flowsthrough the GDE in an aqueous solution. Resistive losses account formost of the potential applied. When using software resistancecompensation (compensated 85% of 1.7 Ohm resistance that was determinedby the software using impedance spectroscopy), the current-potentialcurve shifts, but does not change shape significantly. The high rates ofhydrogen oxidation on GDEs in aqueous solvents demonstrate that it is infact a change of solvent from aqueous to nonaqueous (FIG. 12A) thatleads to poor hydrogen oxidation rates and FEs on carbon fiber GDEs.

FIGS. 17A-17G depict establishment of a gas-liquid boundary in the cell.Panel a shows 20 μL of water on platinum-loaded carbon paper (left,Pt/C) and stainless steel cloth (right, SSC). The water droplet beads upon both materials. Panel b shows 20 μL of 1 M LiBF₄, 0.11 M EtOH, THF(the electrolyte), on Pt/C and SSC. The electrolyte penetrated into thePt/C and spread through it, while the spreading was finite on steelcloth, demonstrating the lack of capillary action on the SSC. FIG. 17Cshows side and front photos of modified cell used to image the gas andelectrolyte compartments. A glass slide is used in place of a PEEK backplate and membrane for the gas and electrolyte compartments,respectively. O-rings are used to evenly distribute pressure on theglass slides, and polycarbonate back plates are used to hold thestructure together. The compartments are imaged without electrolyte (“noelec”), with half the usual volume of electrolyte (“880 μL”), with theusual volume of electrolyte of 1.75 mL and gas flowing through theelectrode (“through”), gas flowing past the electrode at 1 kPa (“flowpast”), and under lack of pressure gradient (“no ΔP”). FIG. 17D showsthe electrolyte compartment when using an SSC. FIG. 17E shows the gascompartment when using an SSC. FIG. 17F shows the electrolytecompartment when using Δt/C. FIG. 17G shows the gas compartment whenusing Pt/C.

FIGS. 18A-18 b depict images of various stages of plating platinum ontosteel cloths. Note that the cloths have been cut into smaller piecesafter metal plating for ease of presentation. FIG. 18A shows steelcloths under glass slides. FIG. 18B shows freely moving steel cloths.Note the brown tint that appears after striking the cloths with nickel,and the darker, black tint that appears after plating platinum onto thenickel.

FIGS. 19A-19F depict scanning electron microscopy (SEM) images of barestainless steel cloths. A Zeiss-Merlin HR-SEM was used to collectimages. FIGS. 19A-19B show low magnification. FIGS. 19C-19D show mediummagnification. FIGS. 19E-19F show high magnification. Panels a, c and eused an Inlens detector, while (b, d, f) used an HE-SE₂ detector.

FIGS. 20A-20F depict scanning electron microscopy (SEM) images ofnickel-coated stainless steel cloths. A Zeiss-Merlin HR-SEM was used tocollect images. FIGS. 20A-20B show low magnification. FIGS. 20C-20D showmedium magnification. FIGS. 20E-20F show high magnification. FIGS. 20A,20C and 20E used an Inlens detector, while FIGS. 20B, 20D and 20F usedan HE-SE₂ detector. Note the uncoated sections at the top of thethreads. They are likely formed when one side of the cloth contactingthe glass wall in the beaker cell used for nickel striking (i.e. frompoor transport of Ni²⁺ to the surface).

FIGS. 21A-21F depict scanning electron microscopy (SEM) images ofsmoothly Pt-coated stainless steel cloths. A Zeiss-Merlin HR-SEM wasused to collect images. FIGS. 21A and 21B show low magnification. FIGS.21C and 21D show medium magnification. FIGS. 21E and 21F show highmagnification. FIGS. 21A, 21C and 21E used an Inlens detector, whileFIGS. 21B, 21D and 21F used an HE-SE2 detector. Note the poorly coatedregions at the top of the threads. As platinum poorly adheres tostainless steel, they are likely caused by the Ni-less regions observedin Ni plated cloths.

FIGS. 22A-22F depicts scanning electron microscopy (SEM) images ofroughly Pt-coated stainless steel cloths. A Zeiss-Merlin HR-SEM was usedto collect images. FIGS. 22A and 22B show low magnification. FIGS. 22Cand 22D show medium magnification. FIGS. 22E and 22F show highmagnification. FIGS. 22A, 22C and 22E use an Inlens detector, whileFIGS. 22B, 22D and 22F used an HE-SE2 detector. Regions of poor platinumcoating are visible as in FIGS. 21A-21F. The platinum coating here doesnot adhere to the fibers as well, leading to rougher surfaces. Themeasured HOR FE is unaffected up to 25 mA cm⁻² of applied current.

FIGS. 23A-23E depict electrochemical characterization of SSC and Pt/SSCelectrodes. A 1 M LiBF₄/0.11 M EtOH/THF electrolyte was used in a3-compartment cell with an Ag/AgCl pseudoreference for all experimentswith 100% software resistance compensation. The resistance was measuredby PEIS. FIG. 23A shows linear sweep voltammograms of Pt/SSC when H₂ orN₂ are flowed past the electrode at 10 sccm and a 1 kPa gradient acrossthe electrode. The high oxidation currents visible when H₂ is fed to theelectrode support the notion that HOR is occurring. FIG. 23B showsimpedance spectra of the system in FIG. 23A. FIG. 23C shows linear sweepvoltammograms of SSC when N₂ or Ar are flowed past the electrode at 10sccm and a 1 kPa gradient across the electrode. FIG. 23D shows impedancespectra of the system in FIG. 23C. FIG. 23E shows constant potentialholds of the system in FIG. 23C. Note that potential overloads in PEISprevented measurement of additional points at lower frequencies. Linearsweep voltammograms were collected at a sweep rate of 5 mV·s⁻¹.

FIGS. 24A-24B depict hydrogen mass balance-based quantification of HORFE. A 1 M LiBF₄/0.11 M EtOH/THF electrolyte was used in a 3-compartmentcell with a Pt/SSC anode for hydrogen oxidation quantification. The timerequired for successive bubbles of gas to leave the gas compartment(FIGS. 32A-32D) was recorded for (a) H₂ and (b) N₂ feed gases before andafter application of 25 mA of current. When H₂ was used as the feed, thetime required for bubbles to evolve decreased when current was appliedbecause H₂ was being consumed at the electrode, while no change wasobserved when N₂ was used as the feed. The FE towards HOR was found tobe 105±2%, i.e. close to unity.

FIGS. 25A-25D depict the effect of flowrate of feed gas past SSC-basedGDEs in electrochemical experiments at an applied current of 25 mA cm⁻².FIG. 25A shows the effect of H₂ flowrate on HOR FE. Note that nosignificant trend in HOR FE is observed when changing the flowrate; theFE is >99% in all cases. FIG. 25B shows the nominal single-passconversion of H₂ as a function of flowrate. At low flowrates (˜0.2sccm), the conversion of H₂ is high; when hydrogen is fed at a ratecorresponding to 100% conversion (0.18 sccm), depletion of hydrogen anda lowering of the pressure gradient across the Pt/SSC is observed, so ahigher flowrate is required to maintain stable operation. FIG. 25C showsthe effect of flowrate on NH₃FE. Note that at lower flowrates, lessammonia is found in the gas phase, while the total amount of ammonia isunchanged with flowrate. FIG. 25D shows the nominal single-passconversion of N₂ as a function of flowrate. The residence time for thegases in the gas compartment range from 8.2 minutes at 0.2 sccm to 10seconds at 10 sccm. Vertical error bars in FIG. 25A represent theuncertainty in the FE measurement for a single run. Vertical error barsin FIGS. 25B and 25D represent the error in conversion, computed fromthe uncertainties in flowrate and FE. Vertical error bars in FIG. 25Crepresent the standard deviation of multiple replicates of the sameexperiment (n>2). Horizontal error bars represent the uncertainty in gasflowrate.

FIGS. 26A-26D depict use of Pt/SSC for hydrogen oxidation in propylenecarbonate-based electrolyte. A 1 M LiBF₄ in 9:1 propylenecarbonate/dimethyl carbonate was used to demonstrate the efficacy ifPt/SSC for hydrogen oxidation. FIG. 26A shows linear sweep voltammograms(LSVs) collected a sweep rate of 5 mV s⁻¹ for flooded and GDEconfigurations. The respective gases (N₂ and H₂) were fed through theelectrolyte at 10 sccm for the flooded (Pt foil) case, or past thePt/SSC at 10 sccm for the GDE (Pt/SSC) case. FIG. 26B shows the samedata as depicted in FIG. 26A, but with a larger range of current valuesshown to demonstrate the high H₂ oxidation current obtained when using aPt/SSC. Higher potentials (and currents) were not possible to applyusing the VMP3 potentiostat, as the total cell voltage exceeded itsoperating range (˜10 V). FIGS. 26C-26D show time intervals between gasbubbles leaving the gas compartment of the 3-compartment cell used inPt/SSC experiments with a propylene carbonate based-electrolyte when(FIG. 26C) H₂ or (FIG. 26D) N₂ was used as the feed gas. The computedFaradaic efficiency towards H₂ oxidation is 112±19%, while it is 5±5%when N₂ is fed to the anode.

FIGS. 27A and 27B depict potentials and Faradaic efficiencies toward NH₃in long duration experiments when using an eHB reactor. FIG. 27A showsthe total cell voltage required for a 20 mA constant current inexperiments utilizing a SCC cathode and a Pt/SSC anode with variousseparators. FIG. 27B shows the Faradaic efficiency toward NH₃ in theaforementioned experiments. Vertical error bars for the Daramic 6 minuteexperiment in FIG. 27B represent the standard deviation of multiplereplicates of the same experiment (n=3). The Faradaic efficiency towardNH₃ decreases from ˜38% in short duration experiments to ˜20% in longerduration experiments, but is fairly independent of the separator used.When a separator (Celgard or Daramic) between the anode and cathodecompartments (FIGS. 31A-31H) is used, the cell voltage increases withtime for unknown reasons. Possible reasons include selective depletionof Li⁺ ions in the catholyte and separator fouling. In an undividedcell, no separator is used, which means that the electrolyte is free toconvect between the electrodes.

FIG. 28 depicts accumulation of ammonia over time in the electrolytewhen using an SSC. The amount of ammonia increases monotonically withtime when 20 mA cm⁻² is applied to a cell with an SSC cathode after aninduction period of approximately 2 minutes. The short induction periodsuggests that the lithium-mediated catalytic cycle reaches steady staterapidly. The amount of ammonia produced is estimated by diluting smallfractions of the catholyte during operation, as described in Nitrogenreduction experiments—time evolution of ammonia. The total amount ofammonia produced in this experiment was measured after the experiment byutilizing the entire catholyte and was found to be 14.5±1 μmol. Verticalerror bars represent a combination of estimates of the error associatedwith ammonia quantification and electrolyte extraction from the cell. Inthis experiment, the production rate and Faradaic efficiency for ammoniaare computed from the slope of the linear fit of data.

FIGS. 29A-29B depict electrical energy losses in the ammonia productionsystem. FIG. 29A shows electrical energy losses at high FE conditions(15 mA cm² applied current, 0.5 kPa pressure gradient across steelcloth). FIG. 29B shows electrical energy losses at high rate conditions(25 mA cm⁻² applied current, 1 kPa pressure gradient across steelcloth). Note that the anode reaction is assumed to be THF oxidation, aswas used in the experiments, hence it is somewhat inefficient. Most ofthe electrical losses are due to high solution resistance. The procedureto calculate these parameters was described below.

FIGS. 30A-30B show depictions of the 3-compartment cell used for GDEexperiments. FIG. 30A depicts a 3D model of the 3-compartment cell usedin GDE experiments. Note: O-rings are not shown. FIG. 30B shows aphotograph of the parts used to assemble a 3-compartment cell. The cellis made of polyether ether ketone (PEEK) plastic. The cell is inspiredby a design used in the CO₂ reduction literature. The cells weremachined in-house; CAD and CAM files are available upon request.

FIGS. 31A-31H depict an assembly of a 3-compartment cell in order. Theprocedure is described above in Assembly of a 3-compartment cell. FIG.31A shows the counter electrode and current collector, here a piece ofplatinum foil and aluminum foil, respectively. FIG. 31B shows thecounter electrolyte compartment. FIG. 31C shows the Daramic polyporousseparator. FIG. 31D shows the working electrolyte compartment. FIG. 31Eshows the gas diffusion electrode, here a stainless steel cloth. FIG.31F shows the aluminum current collector for gas flow. FIG. 31G showsthe gas compartment. FIG. 31H shows the sealed cell.

FIGS. 32A-32D depict a pressure control setup used in gas diffusionelectrode experiments. FIG. 32A shows prior to addition of electrolyteto cell, no pressure gradient across the GDE/SSC is observed as gas ispassed through the GDE/SSC. FIG. 32B shows following addition ofelectrolyte, the pressure in the gas compartment increases until thepressure gradient across the GDE/SSC reaches the Laplace pressure, afterwhich gas continues to pass through the GDE/SSC. FIG. 32C shows thepressure in the gas compartment is lowered, and gas begins to flow pastthe GDE, while the electrolyte stays in the working compartment. FIG.32D shows in NRR experiments, a boric acid trap between the gascompartment and burette is added to capture gas phase ammonia.

FIG. 33 depicts absorbance spectra of diluted samples assayed by thesalicylate method. Calibration absorbance spectra for solutionscontaining 0 and 60 μM of ammonia are shown. The absorbance spectra ofcatholyte diluted to 100 mL and further diluted 2- and 4-fold, asdescribed in the methods, are shown. The measured ammonia concentrationsof the diluted solutions are given explicitly, while the averageconcentration assumes that the concentrations were multiplied by theirrespective dilutions. The experiment was chosen at random; in thisexperiment, 15 mA of current were applied for a total of 7.2 C ofcharge, N₂ was flowed past an SSC electrode in a 3-compartment cell at10 sccm, and the pressure gradient across the SSC was 1.5 kPa. Theresults of this experiment can be found on data line 9 of Table 5.

FIGS. 34A-34D depict typical calibration curves for quantifying ammoniausing the indophenol method. Note that the difference between theabsorbance at 650 nm and 475 nm, called the absorbance signal, is usedto make the calibration curves. FIG. 34A shows absorbance spectra forvarious concentrations of NH₃ in pure water. FIG. 34B shows theresulting calibration curve for NH₃ in pure water. FIG. 34C showsabsorbance spectra for various concentrations of NH₃ in water containing5% of 1 M LiBF₄/THF electrolyte by volume. FIG. 34D shows the resultingcalibration curve for NH₃ in water containing 5% v/v 1 M LiBF₄/THFelectrolyte. Vertical error bars in FIGS. 34B and 34D denote thestandard deviation in absorbance measured between two solutions of equalconcentration. Horizontal error bars in FIGS. 34B and 34D denote theuncertainty in ammonia concentration when preparing solutions. Note thatthe effective extinction coefficient is lower in the presence ofelectrolyte than in pure water; the ammonia concentration is typicallyunderestimated because the pure water calibration curve is used whilesamples contain some electrolyte.

FIGS. 35A-35C depict comparison of NMR and the salicylate assay forammonia quantification. FIG. 35A shows NMR spectra of 3 ammoniacalibration solutions with varied NH₄ ⁺ concentrations in simulatedacidified electrolyte (1.75 mL of 1 M LiBF₄/0.11 M EtOH/THF electrolyte,diluted to 4 mL with 0.05 M H₂SO₄) and a sample of unknown concentrationwith 1 mM maleic acid as an internal standard. The spectra werereferenced to the maleic acid peak, which was chosen to have a chemicalshift of 6.37 ppm. The solution of unknown concentration was obtained byrunning applying 15 mA to a 3-compartment cell with an SSC cathodethrough which 10 sccm of N₂ was flowed. FIG. 35B shows the relative peakareas of the NH₄ ⁺ peaks and the maleic acid peak. FIG. 35C showsmeasured ammonia concentrations via the salicylate assay and therelative intensities of NMR peaks. The measured concentrations from thetwo methods are practically identical if maleic acid is used as aninternal standard. Using BHT in the THF as the internal standardpredicts slightly lower concentrations with large error, likely due tothe smaller concentration of BHT protons in the sample solutions.

FIGS. 36A-36D depict ferrocenium calibration curves for quantifying HORFaradaic efficiency. FIG. 36A shows absorbance spectra for solutions offerrocenium in water at a range of concentrations. FIG. 36B showsabsorbance spectra obtained for solutions involved in HOR controlexperiments. One control experiment involved using N₂ instead of H₂ asthe feed gas, while the others involved applying no current for theduration of the experiment. FIG. 36C shows calibration curves for the255 nm ferrocenium signal using differently prepared solutions. FIG. 36Dshows calibration curves for the 619 nm ferrocenium signal usingdifferently prepared solutions.

FIGS. 37A-37D depict efficiency of the steel cloth-based GDEs for HORand NRR with raw data shown. FIG. 37A shows comparison of HOR Faradaicefficiency (FE) of Pt-coated steel cloths (Pt/SSC) and Pt-loaded carbonpapers (Pt/C) in various configurations. FIG. 37B shows the effect ofpressure gradient across a Pt/SCC on HOR FE at 25 mA cm⁻² appliedcurrent density. FIG. 37C shows the production rate of ammonia as afunction of applied current density on steel cloth cathodes at pressuregradient of 1 kPa across the steel cloth. Solution phase ammonia isfound in the electrolyte while gas phase ammonia in the acid trap afterthe cell. FIG. 37D shows the effect of pressure gradient across a steelcloth cathode on FE toward NH₃ at 15 mA cm⁻² applied current density.Horizontal error bars in FIGS. 37B and 37D represent the range ofpressure gradient values required for gas flow through the SSCs. Thedashed lines in FIGS. 37B and 37D represent the onset of gasbreakthrough in the SSC, which is the Laplace pressure. In allexperiments, 7.2 C of charge were passed to both measure HOR or NRR FE.

FIGS. 38A-38D depict methods used to obtain accurate current andpotential measurements when using a Tekpower DC power source. FIG. 38Ashows a schematic of a circuit to measure current and charge passed.This scheme was used to quantify the charge passed in all experiments.FIG. 38B shows a schematic of a circuit to safely measure and record thetotal potential applied to the cell. This scheme was never actually usedto record the potential, as the DC power source has a digital voltagereadout (FIG. 13D). FIG. 38C shows a typical calibration of a resistorresistance using a VMP3 potentiostat. FIG. 38D shows an examplemeasurement of the voltage drop across the resistor in a 15 mA NRRexperiment and the resulting charge passed.

FIGS. 39A-39D depict evidence of strong Li⁺—NH₃ interactions. FIG. 39Ashows a comparison of ammonia concentrations in ammonia-saturated THFand ammonia-saturated 1 M LiBF₄ in THF. FIG. 39B shows visualdifferences between ammonia-saturated THF with and without LiBF₄. WhenLiBF₄ is present, the solution separates into two phases. It isspeculated that the two phases may be less dense ammonia-saturated THFon top and more dense [Li(NH₃)x] [BF₄] on the bottom. FIG. 39C showsresults of stripping ˜20 mM ammonia in THF with N₂ at 10 sccm for 10minutes. Almost all ammonia left the solution, and was caught in the 0.1M H₃BO₃ trap. FIG. 39D shows results of stripping ˜20 mM ammonia in 1 MLiBF₄ in THF with N₂ at 10 sccm for 10 minutes. Little ammonia (˜2%) wasstripped into the trap, demonstrating the ability of LiBF₄ to preventammonia stripping. Vertical error bars in FIGS. 39A, 39B and 39Drepresent the standard deviation of computed concentrations of samplesolutions; the concentrations were computed by using various dilutionsof the concentrated sample solutions (n≥2).

FIG. 40 depicts the demonstration of facile ferrocene oxidation in theelectrolyte. A linear sweep voltammogram (LSV) measured at a 5 mV s⁻¹sweep rate for an electrolyte containing 10 mM ferrocene, 0.11 M EtOH, 1M LiBF₄ in THF while flowing 10 sccm of N₂ in a 2-compartment cell at aplatinum anode. While thermodynamically ferrocene oxidation is preferredto THF oxidation, it is important to demonstrate that this is accuratekinetically. Here, it can be seen that ferrocene oxidation starts tooccur at potentials slightly below 0 V vs Fc⁺/Fc due to a Nernstianshift, after which it quickly reaches the transport-limited currentdensity. THF oxidation occurs at higher potentials; the exact value ofTHF oxidation was found to be sensitive to platinum anode preparationand age, as well as electrolyte composition. The data were smoothed byusing a 50th percentile smooth filter.

FIG. 41 depicts an electrochemical system.

FIG. 42A depicts a basic standalone electrode architecture. FIG. 42Bdepicts a basic standalone electrode architecture diagram withsee-through edges.

FIG. 43 depicts a flow-in configuration of the standalone electrode.

FIG. 44 depicts a flow-past configuration of the standalone electrode.

FIG. 45 depicts an exemplary electrochemical reaction system utilizing aflow-in and a flow-past standalone GDE electrodes.

FIGS. 46A-46B depict linear sweep voltammograms obtained at 20 mV s-1when using standalone electrodes for in a flow-in configuration hydrogenoxidation in (FIG. 46A) aqueous electrolyte (FIG. 46B) nonaqueous,acetonitrile electrolyte. Applied potentials are IR compensated.

DETAILED DESCRIPTION

A method to utilize sparingly soluble gases in electrochemical reactionsat high rates in nonaqueous solvents is described. The method can berelevant for electroorganic synthesis and fuel production where controlof proton activity is important. The method relies on metallic supportsand a pressure gradient applied across the electrode. The method can beused in a variety of electrochemical systems, for example, as appliedherein to hydrogen oxidation in two nonaqueous solvents and nitrogenreduction in one solvent; the two chemistries are coupled to produceammonia from nitrogen and hydrogen at high rates.

A sparingly soluble gas, generally, is a non-polar gas that does notreact or interact favorably with a solvents. For example, N₂, H₂, CO,and CH₄ are sparingly soluble. Gasses with solubilities less than 50-100mM at 1 atm can be considered to be a sparingly soluble gas. A gas witha Henry's constant <0.05 M/atm can be considered to be sparingly solublegas.

An electrochemical system can include a housing including a chamber, anelectrode within the housing, and a gas permeable metal on a surface ofthe electrode in contact with the chamber. The system can be used in amethod of supplying a gas to an electrochemical system can includecontacting a gas with a gas permeable metal on a surface of an electrodein a chamber of a housing. The method can include applying a voltage tothe electrode. The system can be a gas diffusion electrochemical system,in which metallic supports can be used in the GDEs combined with apressure gradient across the GDE. The gas can be at a higher pressurerelative to the liquid, to obtain effective gas-liquid contacting at theelectrode surface for high reaction rates in nonaqueous solvents.Metallic supports can avoid flooding of the electrodes in the absence ofhydrophobic repulsion, while the pressure gradient helps maintain thegas-liquid interface in the desired location. As the method relies on aphysical effect (a pressure gradient) to establish the gas-liquidboundary, it can be used with any solvent, including nonaqueoussolvents.

The gas permeable metal can be a metal support, which can include metalfibers or a porous metal. The metallic support can be stainless steel,which was be woven into a fine cloths with very thin fibers. In oneexample, the cloth can be a 400×400 mesh, which contains 400 fibers 25micrometers in diameter per inch of length, with a spacing ofapproximately 25-40 micrometers between fibers. The gas permeable metalcan include openings of between 1 and 200 micrometers, preferablybetween 2 and 100 micrometers. The metallic supports can be made fromany metal that is amenable to forming, including, but not limited tostainless steel (304 and 316), steel, nickel, iron, copper, silver,gold, or platinum. The metals can be formed into porous materials whichare gas permeable, such as metal cloths and meshes, but also metalfilters and sponges. The characteristic pore size of the material can beat least as large as 200 micrometers, and down to 2 micrometers; thepore size can be smaller if larger operating pressure gradients aredesired. In certain circumstances, the gas permeable metal can include ametal mesh system. The metal mesh can be symmetric or asymmetric. Incertain circumstances, the metal mesh can include 100, 200, 300, 400 or500 fibers per inch. In other circumstances, the metal mesh can beasymmetric and include 100, 200, 300, 400 or 500 fibers per inch in onedirection and 500, 1000, 1500, or 2000 fibers per inch in a seconddirection. In certain circumstances, the gas permeable metal can beexposed to a pressure gradient. In certain circumstances, the method caninclude supplying a pressure of the gas in the chamber to create apressure differential in the housing. The pressure gradient appliedacross the cloth can be 0.5 to 10 kilopascals, for example, 1, 2, 3, 4or 5 kilopascals. The pressure gradient can depend on the electrolyteused and the pore size of the support. At high pressure gradients, abovethe Laplace pressure of the material, gas may cross the support andenter the electrolyte; the invention still works even under theseoperating conditions.

The gas permeable metal can include additional catalysts grown, placed,or deposited on a surface of the metal. Potential catalysts can include:metals, such as silver, gold, platinum, nickel, lithium, zinc, ortitanium; metal oxides, such as iridium oxide, cobalt oxide, iron oxide,copper oxide, titanium oxide, or silver oxide; metal sulfides, such asmolybdenum sulfide, or cadmium sulfide; metal nitrides, such as lithiumnitride, cobalt nitride, nickel nitride, and mixtures thereof; metalphosphides, such as cobalt phosphide, nickel phosphide, or mixturesthereof; molecular catalysts, such as metal phthalocyanines, such ascobalt phthalocyanine or metal porphyrins.

Catalysts can be deposited onto the metal substrate or synthesized onits surface. Methods for deposition include electroplating of metals,electroless plating of metals, electrophoretic deposition, sputtering,pulse laser deposition, chemical vapor deposition, spin-coating, orapplication of catalyst inks. Methods for in-situ manufacture includeoxidation (for making oxides), treatment with nitrogen and ammonia (formaking nitrides), heating with sulfur (for making sulfides), heatingwith phosphorus (for making phosphides), or thermal decomposition ofcomplex materials.

Referring to FIG. 41, gas diffusion electrochemical system 10 includes ahousing 12, a first electrode 16, and a second electrode 20. A voltagecan be applied to the first electrode and the electrode. One or more ofthe first electrode or the second electrode, or both, can include acatalyst composition. Substrate inlet 14 can be used to introduce a gasspecies that will be oxidized or reduced, such as nitrogen, oxygen,hydrogen, carbon monoxide or carbon dioxide gas into the housing 12. Thegas species in housing 12 can be pressurized relative to the electrolytefluid 25. The first electrode 16 can include a gas permeable metalconfigured to contact with the gas species. The half-reaction takingplace in housing 12 involves the gas species. The second electrode 20 inhousing 18 is opposite first electrode 16. The second electrode 20 can,optionally, include a second catalyst composition. A separator 30 can bepresent between first electrode 16 and second electrode 20. Anelectrolyte fluid 25 can be positioned between the first electrode 16and the second electrode 20. The electrolyte fluid can include anaqueous solvent or a non-aqueous solvent or a water-containingnon-aqueous solvent. Depending on the circumstances, the non-aqueoussolvent can include 0%, 0.2%, 0.5%, 1.0%, 5%, or 10% water. The solutioncan flow through housing 12, including the electrochemical product, canbe carried out of outlet 40. A half-reaction taking place in housing 18to generate a second electrochemical product can involve a second gasspecies that enters housing 18 at port 15 and the product can exit thehousing 18 through port 30. Housing 12 and housing 18 can be separatestructures or can form a single structure. A pressure gradient can existbetween the gas-containing housing, housing 12, and the electrolytefluid 25. A pressure gradient can exist between the gas-containinghousing, housing 18, and the electrolyte fluid 25.

The gas species can be a gas species that can be oxidized or reduced,for example, N₂, O₂, H₂, CO or CO₂.

The solvent can be an inert organic solvent that in which theelectrolyte salt, substrate, and proton carrier can be dissolved. Incertain circumstances, a carbonylation reaction or reductive aminationreaction can involve a substrate dissolved in the solvent. Theconcentration of the proton carrier can be 5 mM, 10 mM, 15 mM, 20 mM, 25mM, 30 mM, 35 mM, 40 mM, 50 mM, 60 mM, 70 mM, 80 mM, 90 mM, 100 mM, 200mM, 250 mM, 300 mM, 350 mM, 400 mM, 450 mM, or 500 mM. For example, fornitrogen reduction, the concentration of the proton carrier can be 50 mMor higher. The concentration of the substrate can be 5 mM, 10 mM, 15 mM,20 mM, 25 mM, 30 mM, 35 mM, 40 mM, 50 mM, 60 mM, 70 mM, 80 mM, 90 mM,100 mM, 200 mM, 250 mM, 300 mM, 350 mM, 400 mM, 450 mM, or 500 mM. Forexample, for nitrogen reduction, the concentration of the substrate canbe 50 mM or higher. The concentration of the electrolyte can be 0.1 M,0.25 M, 0.5 M, 1 M, 2 M, 4 M, 5 M, 6 M, 7 M, 8 M, 9 M, or 10 M.

The temperature and pressure can be ambient temperature and pressure.There can be a pressure gradient between housing 12 and electrolytefluid 25 and housing 18 and electrolyte fluid 25.

The reaction product can be produced in a gas phase.

The voltage can be between about 0.2V and 40.0V, between about 0.4V and35.0V, or between about 1V and 30.0V. For example, the voltage can beabout 1.0V, 5.0V, 10.0V, 15.0V, 20.0V, 25.0V, 30.0V, 35.0V, or 40.0V.

Each of the first electrode and the second electrode can be or caninclude a noble metal, for example, platinum or palladium.

A variety of reactor designs can implement the method. The methoddescribed herein can be performed under various differentelectrochemical cell geometries and configurations, which include ananode and cathode, connected to an external power source, with anionically conductive medium between the two electrodes. A thirdreference electrode may be incorporated if necessary for control of thepotential at the electrodes. Resistive losses can be reduced bydecreasing the distance between electrodes. The process may be conductedunder batch or continuous conditions. An ionically conductive membrane,such as Nafion or Selemion, or a separator, such as Celgard or Daramic,can be used in the structure, but is not required.

The overall reaction may be tuned by choice of the cathode and thereactor conditions. For instance, if nitrogen is flowed to the cathode,then nitrogen will be reduced to generate ammonia; if hydrogen is flowedto the anode, then hydrogen will be reduced to generate protons.

The nonaqueous solvent can include acetonitrile, DMSO (dimethylsulfoxide), DMF (dimethylformamide), THF (tetrahydrofuran), DCM(dichloromethane), and propionitrile. The electrolyte can contain aconductive salt such as TBABF₄ (Tetrabutylammonium tetrafluoroborate),TBAPF₆ (Tetrabutylammonium hexafluorophosphate), NaClO₄(Sodiumperchlorate), LiClO₄(Lithium perchlorate), or TEAP(tetraethylammoniumperchlorate), or a combination thereof. The non-aqueous solvent canassist substrate solubility.

The electrodes can include a catalyst. Catalysts that may be used inthese GDEs include metals such as alkali metals such as lithium, sodium,potassium, alkali-earth metals such as magnesium, transition metals suchnickel, platinum, copper, gold, silver. Metallic catalysts can bedeposited onto the supports electrochemically from solution, viaelectroless plating, or sputtered onto the supports ex situ. Catalystnanoparticles such as metal oxides, metal nitrides, and metal sulfidescan be deposited onto the supports via drop casting, sputtering, orpulse laser deposition. For nitrogen reduction, lithium metal can bedeposited in situ electrochemically. For hydrogen oxidation, nickel canbe deposited electrochemically onto stainless steel cloths, onto whichplatinum is then electrochemically deposited.

Electrochemical transformations in nonaqueous solvents are important forsynthetic and energy storage applications. Use of nonpolar gaseousreactants such as nitrogen and hydrogen in nonaqueous solvents islimited by their low solubility and slow transport. Conventional gasdiffusion electrodes improve transport of gaseous species in aqueouselectrolytes by facilitating efficient gas-liquid contacting in thevicinity of the electrode. Their use with nonaqueous solvents ishampered by the absence of hydrophobic repulsion between the liquidphase and carbon fiber support. Herein, a method to overcome transportlimitations in tetrahydrofuran is reported using a stainless steelcloth-based support for ammonia synthesis paired with hydrogenoxidation. An ammonia partial current density of 8.7±1.5 mA cm⁻² and aFaradaic efficiency of 35±6% are obtained using a lithium-mediatedapproach. Hydrogen oxidation current densities up to 25 mA cm⁻² areobtained in two nonaqueous solvents with nearly unity Faradaicefficiency. The approach can be applied to produce ammonia from nitrogenand water splitting-derived hydrogen.

Electrochemical synthesis of chemicals is an attractive alternativeapproach to traditional thermochemical methods. In some reactions,electric potential can act as a thermodynamic driving force instead ofhigh temperatures and pressures, which may allow for operation at milderconditions and in a modular fashion. See, for example, Schiffer, Z. J. &Manthiram, K. Electrification and Decarbonization of the ChemicalIndustry. Joule 1, 10-14 (2017); and Yan, M., Kawamata, Y. & Baran, P.S. Synthetic Organic Electrochemistry: Calling All Engineers. Angew.Chemie Int. Ed. 57, 4149-4155 (2018), each of which is incorporated byreference in its entirety. Ammonia (NH₃) production is an example of areaction that may benefit from being operated electrochemically. See,for example, Chen, J. G. et al. Beyond fossil fuel-driven nitrogentransformations. Science (80). 360, eaar6611 (2018); and Soloveichik, G.Electrochemical synthesis of ammonia as a potential alternative to theHaber-Bosch process. Nat. Catal. 2, 377-380 (2019), each of which isincorporated by reference in its entirety. NH₃ is currently producedpredominantly via the Haber-Bosch process, which operates at hightemperatures (300-500° C.) and pressures (200-300 bar) and requires acoupled steam reforming plant for hydrogen (H₂) production. See, forexample, Shipman, M. A. & Symes, M. D. Recent progress towards theelectrosynthesis of ammonia from sustainable resources. Catal. Today286, 57-68 (2017), which is incorporated by reference in its entirety.This leads to high capital costs for the process and centralization ofproduction, a situation that is poorly matched with the distributednature of ammonia utilization. See, for example, Comer, B. M. et al.Prospects and Challenges for Solar Fertilizers. Joule 3, 1578-1605(2019) which is incorporated by reference in its entirety. Alternativemethods for producing hydrogen, such as water splitting, may overcomesome of the issues associated with the traditional Haber-Bosch process,such as the large amount of CO₂ emissions and high capital costassociated with steam reforming. See, for example, Suryanto, B. H. R. etal. Challenges and prospects in the catalysis of electroreduction ofnitrogen to ammonia. Nat. Catal. 2, 290-296 (2019), which isincorporated by reference in its entirety. However, these methods do notovercome the need for large scales for the ammonia synthesis reactoritself, as it must still be run at high temperatures and pressures. Anelectrochemical process—even one which utilizes multiple reactors—thatproduces ammonia from nitrogen and water requires a thermodynamicminimum potential of 1.17 V at standard conditions. See, for example,Soloveichik, G. Electrochemical synthesis of ammonia as a potentialalternative to the Haber-Bosch process. Nat. Catal. 2, 377-380 (2019),which is incorporated by reference in its entirety. Potential is apotent thermodynamic driver, providing mild conditions conducive tomodular and small-scale operation of electrochemical processes. See, forexample, Foster, S. L. et al. Catalysts for nitrogen reduction toammonia. Nat. Catal. 1, 490-500 (2018), which is incorporated byreference in its entirety.

Despite the attractiveness of electrochemistry in syntheticapplications, several challenges must be overcome to allow for efficientscale-up of the technology. One of the most important issues is the useof non-renewable reactants at the counter electrode. For reductivechemistries, which are important for energy storage and certainsynthetic applications, the counter reaction is often oxidation ofsolvent or sacrificial anodes made of active metals. See, for example,Jiao, F. & Xu, B. Electrochemical Ammonia Synthesis and Ammonia FuelCells. Adv. Mater. 31, 1805173 (2019); Davis, S. J. et al. Net-zeroemissions energy systems. Science (80). 360, eaas9793 (2018); Liu, X.,Jiao, Y., Zheng, Y., Jaroniec, M. & Qiao, S.-Z. Building Up a Picture ofthe Electrocatalytic Nitrogen Reduction Activity of Transition MetalSingle-Atom Catalysts. J. Am. Chem. Soc. 141, 9664-9672 (2019); Peters,B. K. et al. Scalable and safe synthetic organic electroreductioninspired by Li-ion battery chemistry. Science (80-.). 363, 838-845(2019); Matthessen, R., Fransaer, J., Binnemans, K. & De Vos, D. E.Electrocarboxylation: towards sustainable and efficient synthesis ofvaluable carboxylic acids. Beilstein J. Org. Chem. 10, 2484-2500 (2014);and Motile, S. et al. Modern Electrochemical Aspects for the Synthesisof Value-Added Organic Products. Angew. Chemie-Int. Ed. 57, 6018-6041(2018), each of which is incorporated by reference in its entirety. Inaqueous systems, solvent oxidation is permissible and often desired.However, oxidation of organic solvents or sacrificial anodes decreasesthe atom economy of reactions greatly and makes processes utilizingthese reactions poorly amenable to continuous operation. See, forexample, Matthessen, R., Fransaer, J., Binnemans, K. & De Vos, D. E.Electrocarboxylation: towards sustainable and efficient synthesis ofvaluable carboxylic acids. Beilstein J. Org. Chem. 10, 2484-2500 (2014),which is incorporated by reference in its entirety.

In the context of nitrogen reduction, one method that produces ammoniaat high rates and Faradaic efficiencies is the lithium-mediatedapproach. The approach involves reacting lithium metal with nitrogen toform lithium nitride, a reaction which is spontaneous at ambientconditions. The lithium nitride is then protonated to make ammonia and alithium salt. The lithium salt is electrochemically reduced to lithiummetal to close the catalytic cycle (FIG. 10A). The efficacy of thechemistry has been demonstrated in batch processes in which theaforementioned reactions are run with temporal separation; mostly theydiffer in the method used to generate lithium metal. See, for example,McEnaney, J. M. et al. Ammonia synthesis from N₂ and H₂O using a lithiumcycling electrification strategy at atmospheric pressure. EnergyEnviron. Sci. 10, 1621-1630 (2017); Kim, K., Chen, Y., Han, J.-I., Yoon,H. C. & Li, W. Lithium-mediated ammonia synthesis from water andnitrogen: a membrane-free approach enabled by an immiscibleaqueous/organic hybrid electrolyte system. Green Chem. (2019)doi:10.1039/C9GC01338E; Kim, K. et al. Electrochemical Synthesis ofAmmonia from Water and Nitrogen: A Lithium-Mediated Approach UsingLithium-Ion Conducting Glass Ceramics. ChemSusChem 11, 120-124 (2018);and Kim, K. et al. Lithium-Mediated Ammonia Electro-Synthesis: Effect ofCsClO 4 on Lithium Plating Efficiency and Ammonia Synthesis. J.Electrochem. Soc. 165, F1027-F1031 (2018), each of which is incorporatedby reference in its entirety. While these processes demonstrate highFaradaic efficiencies, they are not directly amenable to continuousammonia production, though approaches to utilize rotating reactors for apseudo-continuous process have been proposed. See, for example,McEnaney, J. M. US20180029895A1, which is incorporated by reference inits entirety. In this regard, continuous processes in which all threereactions happen simultaneously are attractive. Typically, continuousprocesses utilize a lithium salt in tetrahydrofuran (THF) electrolytewith ethanol as a proton source. See, for example, Lazouski, N.,Schiffer, Z. J., Williams, K. & Manthiram, K. Understanding ContinuousLithium-Mediated Electrochemical Nitrogen Reduction. Joule 3, 1127-1139(2019); Andersen, S. Z. et al. A rigorous electrochemical ammoniasynthesis protocol with quantitative isotope measurements. Nature 570,504-508 (2019); Tsuneto, A., Kudo, A. & Sakata, T. Lithium-mediatedelectrochemical reduction of high pressure N₂ to NH₃ . J. Electroanal.Chem. 367, 183-188 (1994); and Schwalbe, J. A. et al. A CombinedTheory-Experiment analysis of the Surface Species in Lithium MediatedNH₃Electrosynthesis. ChemElectroChem celc.201902124 (2020)doi:10.1002/celc.201902124, each of which is incorporated by referencein its entirety. While the cathode reactions in this system arewell-described (FIG. 10A), the anode reaction is poorly defined andlikely involves THF oxidation. Solvent decomposition prevents the methodfrom being a practical approach to ammonia production.

Oxidizing H₂ at the anode to produce protons of a controlledthermodynamic activity avoids the aforementioned issues. See, forexample, Singh, A. R. et al. Strategies toward Selective ElectrochemicalAmmonia Synthesis. ACS Catal. 9, 8316-8324 (2019), which is incorporatedby reference in its entirety. As an added benefit, hydrogen oxidationcan be used as a renewable anode reaction for synthetic applications inwhich sacrificial anodes are used, allowing for continuous production ofuseful chemicals. See, for example, Matthessen, R., Fransaer, J.,Binnemans, K. & De Vos, D. E. Electrocarboxylation: towards sustainableand efficient synthesis of valuable carboxylic acids. Beilstein J. Org.Chem. 10, 2484-2500 (2014), which is incorporated by reference in itsentirety. However, the rate of hydrogen oxidation in nonaqueous solventsat flooded electrodes is limited by the solubility of hydrogen and itscorresponding diffusion-limited oxidation rate, equal to severalmilliamperes per square centimeter (mA cm⁻²) (FIG. 10B), which is fartoo low for practical applications. See, for example, Gibanel, F.,Lopez, M. C., Royo, F. M., Santafé, J. & Urieta, J. S. Solubility ofnonpolar gases in tetrahydrofuran at 0 to 30° C. and 101.33 kPa partialpressure of gas. J. Solution Chem. 22, 211-217 (1993); and Bard, A. J. &Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications.(John Wiley & Sons, Inc, 2001), each of which is incorporated byreference in its entirety. Similar diffusion limitations are pronouncedfor gaseous feedstocks such as N₂ (FIG. 10C). See, for example,Lazouski, N., Schiffer, Z. J., Williams, K. & Manthiram, K.Understanding Continuous Lithium-Mediated Electrochemical NitrogenReduction. Joule 3, 1127-1139 (2019); and Zhou, F. et al.Electro-synthesis of ammonia from nitrogen at ambient temperature andpressure in ionic liquids. Energy Environ. Sci. 10, 2516-2520 (2017),each of which is incorporated by reference in its entirety.

One way to overcome diffusion limitations for gaseous reactants inelectrochemical reactions is to use gas diffusion electrodes (GDEs), inwhich intimate contact between the gas, electrolyte, and catalyst isgenerated. See, for example, Mathur, V. & Crawford, J. Fundamentals ofGas Diffusion Layers in PEM Fuel Cells. Recent Trends Fuel Cell Sci.Technol. 400, 116-128 (2007), which is incorporated by reference in itsentirety. This contacting minimizes the distance that gas molecules haveto travel through the electrolyte to react at the catalyst (FIG. 11A),thus achieving much higher diffusion rates than are possible at floodedelectrodes in aqueous electrolytes. GDEs have been used in hydrogen fuelcells and for CO and CO₂ reduction. See, for example, Mathur, V. &Crawford, J. Fundamentals of Gas Diffusion Layers in PEM Fuel Cells.Recent Trends Fuel Cell Sci. Technol. 400, 116-128 (2007); Litster, S. &McLean, G. PEM fuel cell electrodes. J. Power Sources 130, 61-76 (2004);Ripatti, D. S., Veltman, T. R. & Kanan, M. W. Carbon Monoxide GasDiffusion Electrolysis that Produces Concentrated C₂ Products with HighSingle-Pass Conversion. Joule 3, 240-256 (2018); Ren, S. et al.Molecular electrocatalysts can mediate fast, selective CO₂ reduction ina flow cell. Science (80-.). 365, 367-369 (2019); Higgins, D., Hahn, C.,Xiang, C., Jaramillo, T. F. & Weber, A. Z. Gas-Diffusion Electrodes forCarbon Dioxide Reduction: A New Paradigm. ACS Energy Lett. 4, 317-324(2019); and Burdyny, T. & Smith, W. A. CO₂ reduction on gas-diffusionelectrodes and why catalytic performance must be assessed atcommercially-relevant conditions. Energy Environ. Sci. 12, 1442-1453(2019), each of which is incorporated by reference in its entirety.

In the aforementioned applications, the electrolytes are typicallyaqueous solutions or water-saturated polymeric materials, while the GDEsupport is hydrophobized to control wetting. See, for example, Mathur,V. & Crawford, J. Fundamentals of Gas Diffusion Layers in PEM FuelCells. Recent Trends Fuel Cell Sci. Technol. 400, 116-128 (2007), whichis incorporated by reference in its entirety. The hydrophobicinteractions between the electrolyte and support, as well as the smallpore size in the support prevent electrolyte penetration and floodinginto the fibrous structure of the GDE. Instead, a thin layer ofelectrolyte is in contact with the catalyst through which reactant gasmolecules must diffuse (FIG. 11A). See, for example, Weng, L. C., Bell,A. T. & Weber, A. Z. Modeling gas-diffusion electrodes for CO₂reduction. Phys. Chem. Chem. Phys. 20, 16973-16984 (2018), which isincorporated by reference in its entirety. If the primary component ofthe electrolyte is a nonaqueous solvent, such as tetrahydrofuran (THF),then the interactions between the support and electrolyte are no longerunfavorable, which leads to penetration of the electrolyte into thefibrous structure of the GDE (FIG. 11B), effectively flooding thecatalyst. The gas must then diffuse over large distances through thesolution to react (FIG. 11B), lowering the maximum obtainable currentdensities. See, for example, Tran, C., Yang, X.-Q. & Qu, D.Investigation of the gas-diffusion-electrode used as lithium/air cathodein non-aqueous electrolyte and the importance of carbon materialporosity. J. Power Sources 195, 2057-2063 (2010), which is incorporatedby reference in its entirety. Flooding prevents the use of standardcarbon fiber-based GDEs with nonaqueous electrolytes for improving therates of reactions involving sparingly soluble gases. While someapproaches to overcome these issues have been reported, no previouslyreported GDE generated effective gas-liquid contact to greatly increasethe obtained current for electrosynthetic applications in nonaqueoussolvents. See, for example, Tran, C., Yang, X.-Q. & Qu, D. Investigationof the gas-diffusion-electrode used as lithium/air cathode innon-aqueous electrolyte and the importance of carbon material porosity.J. Power Sources 195, 2057-2063 (2010); Balaish, M., Kraytsberg, A. &Ein-Eli, Y. Realization of an artificial three-phase reaction zone in aLi-Air battery. ChemElectroChem 1, 90-94 (2014); and Gourdin, G., Xiao,N., McCulloch, W. & Wu, Y. Use of Polarization Curves and ImpedanceAnalyses to Optimize the ‘triple-Phase Boundary’ in K—O₂ Batteries. ACSAppl. Mater. Interfaces 11, 2925-2934 (2019), each of which isincorporated by reference in its entirety. As described herein, GDE-likebehavior was obtained by controlling material wetting and electrolytepenetration into supports when using nonaqueous electrolytes. Astainless steel cloth (SSC) was used as the substrate onto which thecatalyst was deposited. The electrolyte penetration was controlled bymaintaining a pressure gradient across the cloth. The approach was usedto efficiently oxidize H₂ on Pt-coated steel cloths (Pt/SSC) at currentdensities up to 25 mA cm⁻² in tetrahydrofuran and propylenecarbonate-based electrolytes. In addition, by using an SSC as asubstrate onto which lithium metal is plated in situ, it was possible toreduce N₂ to NH₃ using a lithium-mediated approach. An NH₃ partialcurrent density of 8.7±1.5 mA cm⁻² and high FEs (35±6% at rate-optimizedconditions, 47.5±4% at FE-optimized conditions) were obtained. The twoelectrodes were coupled to build an electrochemical Haber-Bosch (eHB)reactor, which can produce NH₃ from N₂ and H₂ at ambient conditions.

An electrochemical Haber-Bosch reactor is described below, wherehydrogen and nitrogen are utilized to produce ammonia at ambientconditions using electrical potential. The electrolyte contains onemolar tetrafluoroborate (1 M LiBF₄) and 0.11 molar ethanol (EtOH) intetrahydrofuran (THF); it is nonaqueous, as water is not used as thebulk solvent for the electrolyte. Nitrogen gas is reduced at the cathodeon a stainless steel cloth by lithium metal which is electrochemicallyplated onto the mesh in situ. The steel cloth is set up to act as gasdiffusion electrode, with the electrolyte and gas well-separated by thesloth, which generates efficient gas-liquid contacting at the electrode.The steel mesh acts as the support onto which the catalyst, lithiummetal, is deposited in situ. Nitrogen gas was flowed past the electrode,with single pass conversions reaching ˜10%.

At the anode, the electrode was a platinum-coated stainless steel cloththat was also set up as a gas diffusion electrode. Hydrogen gas wasoxidized at anode with nearly unity (>99%) Faradaic efficiency and ratesat least an order of magnitude higher than possible at floodedelectrodes (˜2.5 mA cm⁻² at flooded electrodes vs 25 mA cm⁻² at theGDE). High single pass conversions (>80%) of the feed gas weredemonstrated.

In addition to demonstrating the ability to oxidize hydrogen and reducenitrogen in THF-based electrolytes, the work also demonstrates thatplatinum coated stainless steel cloths can be used to oxidize hydrogenin other nonaqueous solvents. The solvent used to demonstrate this was 1M LiBF₄ in 9:1 propylene carbonate/ethylene carbonate. In this solvent,the transport-limited current density for hydrogen oxidation atconventional flooded platinum electrodes is ˜0.25 mA cm⁻², fartoo lowfor practical applications. Oxidation was demonstrated with unityselectivity at rates two orders of magnitude higher than previouslyachieved, at 25 mA cm⁻².

The architecture described herein can also be used to improve theselectivities and rates of fuel production from gaseous feedstocks suchas N₂, CO, and CO₂ by alleviating diffusion limitations and allowing forprecise control of proton activity in a nonaqueous solvent. For example,the hydrogen oxidation anode described herein can also be coupled to awide range of cathodic hydrogenations, making it broadly useful inelectroorganic synthesis, an emerging area that is finding commercialinterest in energy and pharmaceutical companies. The methods and systemsdescribed herein currently holds the record for the highest rates ofammonia synthesis at ambient conditions (FIG. 3). For example, theammonia production rate can be greater than 1×10⁻⁸ mol cm⁻²s⁻¹. TheFaradaic efficiency can be greater than 30%, preferably greater than40%.

As electrochemical reactions involving poorly soluble gases at floodedelectrodes are limited by the diffusion rate of gas molecules from thebulk electrolyte to the electrode surface, methods to decrease thediffusion distance have been proposed. In gas diffusion electrodes, theinterface between the gas and liquid phases is positioned close to theelectroactive surface to significantly reduce the distance that the gasmust travel, thus increasing rates. In traditional carbon-fiber gasdiffusion electrodes, which are used with aqueous electrolytes, thecarbon fibers are hydrophobic; the hydrophobic repulsion between theelectrolyte and carbon fibers, as well as the small spacing (pore size)between the fibers prevents catalyst flooding by the electrolyte. Thus,a well-defined boundary between the gas and liquid close to the catalystis obtained. Hydrophobic coatings of carbon fibers with nonaqueouselectrolytes do not allow for development of a well-defined gas-liquidinterface as the interactions between the carbon fiber and electrolyteand no longer unfavorable, and sometimes favorable, leading to floodingof the catalyst and increases in the distance that molecules need todiffuse.

To understand where the liquid-gas interface is relative to the solidsurface, one can look at the relative energies of the gaseous and liquidphases under various conditions. First, assume that the electrode isparallel to the gravitational vector; some modification of the analysiscan be made to accommodate angled electrodes. Next, find theconfigurations of gas and liquid that are energetically stable ormeta-stable to determine the location of the gas-liquid interface. Forthese analyses, assume that the system is closed and that the amount ofgas or liquid in it is constant. Transitions between states require thegas and liquid to move around, i.e. exchange places. The total energy inthe system can be quantified as follows:

$E = {\int{\left( {P + {\rho\;{gy}} + \frac{\rho\; v^{2}}{2}} \right)dV}}$

Here, P is the pressure at a given location, ρ is the density of thephase at a given location, 9 is the gravitational acceleration constant,y is the vertical location, v is the velocity of the phase at a givenlocation. The below analysis will assume stagnant phases, so v=0. Onecan note from the outset that the liquid phase is more affected by thegravitational force than the gas phase, while both phases are affectedby the pressure. These heuristics can be used to predict qualitativelythe configurations of gas-liquid boundaries and their stability. Forease of analysis, one can also plot an average energy as a function ofposition along the x-axis:

${E(x)} = {{\int{\left( {P + {\rho\;{gy}}} \right){dy}}} \approx {{P \cdot H_{GDE}} + \frac{\rho_{liquid}gH_{liquid}^{2}}{2}}}$

In the simplest case, take a box only half-filled with liquid. Thestable configuration is one where the liquid is all below the gas, as itis denser, irrespective of initial configuration, shown in FIG. 4. If animpenetrable wall, such as a solid electrode, is place in the box, thestable, equilibrium configuration does not change, shown in FIG. 5.However, for any initial configuration, there is a metastableconfiguration in which the liquid is level in each one of thecompartments, but not equal between compartments, shown in FIG. 6. Goingfrom the metastable to the equilibrium state is hampered by a largeenergy barrier for the liquid phase, as the liquid cannot penetratethrough the wall. In these systems, the gas-liquid interface ishorizontal, while the electrode is vertical, so the amount of electrodearea with good gas-liquid-solid contacting is small. In theory,electrodes could be positioned horizontally for GDE-like behavior in astable manner, shown in FIG. 7. Practically, this may be difficult toimplement and scale because of the nature of the gas-liquid interface atthe cathode and the anode.

A traditional hydrophobic gas diffusion electrode with an aqueouselectrolyte. In this case, the barrier (wall) is porous, so the waterand gas could, in theory, reach the equilibrium state in FIG. 5.However, the liquid phase (water) cannot move freely though the porouselectrode because of the hydrophobic interactions between the substrateand electrolyte. This effect is manifested as a pressure gradient acrossthe interface between the gas phase and the liquid inside the porousstructure, roughly given by the Young-Laplace equation:

${P_{liquid} - P_{gas}} = \frac{2\gamma_{SL}}{R_{pore}}$

The gas similarly cannot enter the liquid phase, as forming the firstbubble is also limited by the Laplace pressure:

${P_{gas} - P_{liquid}} = {\frac{2\gamma_{GL}}{R_{bubble}} \approx \frac{2\gamma_{GL}}{R_{pore}}}$

Therefore there are two energy barriers, one for the liquid, one for thegas, which maintains a meta-stable boundary between the gas and liquidat the GDE, which allows for increased rates. When nonaqueouselectrolyte is used, the energy barrier for the liquid phase penetrationis broken, which leads to electrolyte penetration, catalyst flooding,and lower rates of gas utilization. In some cases, the capillary actioncan make the energy inside the electrode lower than outside of it(carbon fibers+THF, for instance), which exacerbates flooding. See, FIG.8. To prevent electrolyte penetration, one can artificially impose anenergy barrier for the liquid phase. As described herein, this is doneby increasing the pressure in one of the compartments (the gascompartment). While this forces both phases to the lower energycompartment, the gas cannot enter it due to the Laplace pressure; theliquid phase is stably in the other compartment. See, for example, FIG.9. In this case, the gas phase is metastable, while the liquid phase isin its lowest energy state, as long as

P _(gas) −P _(liquid) >ρgH

This requirement imposes a maximum height of GDE that can be used. Theheight can be increased by decreasing the pore size of the GDE, whichincreases the Laplace pressure of the GDE, which in turn increases thehighest allowable pressure gradient. Through this analysis,surprisingly, both traditional carbon fiber GDEs and metal mesh GDEswith a pressure gradient can both utilize local energy minima for phasedistributions to obtain gas-liquid interfaces close to the electrodesurface for increased rates. In carbon fiber GDEs, stability is obtainedby a “phantom” pressure gradient from hydrophobic interactions, while inmetal mesh GDEs, the pressure gradient can be explicitly applied.

Results

Carbon fiber electrodes for hydrogen oxidation

First, SSCs were examined to improve the rates of the hydrogen oxidationreaction (HOR) in a THF-based electrolyte in order to utilize it as arenewable anode chemistry. Initially, commercially available platinum oncarbon fiber (Pt/C) GDEs were used, which are capable of greatlyincreasing the rate of hydrogen oxidation in aqueous electrolytes (FIG.16). When flowing H₂ gas past the GDEs with a THF-based electrolyte itwas found that very little H₂ oxidation occurs at any applied current(FIG. 12A). Establishing a large (20 kPa) pressure gradient across theGDE to allow gas flow through the GDE in order to prevent completeflooding improved HOR FE somewhat (FIG. 12A). The pressure gradient atwhich flow through the GDE is observed is defined by the Laplacepressure; it was found to be 20±4 kPa for Pt/C GDEs. At pressuregradients at which gas flow through the electrode is observed,commercially available Pt/C GDEs were able to support HOR partialcurrent of ˜12 mA cm⁻² (FIG. 12A), estimated by taking the product ofthe total applied current and measured HOR FE. See, for example,Santamaria, A. D., Das, P. K., MacDonald, J. C. & Weber, A. Z.Liquid-water interactions with gas-diffusion-layer surfaces. J.Electrochem. Soc. 161, F1184-F1193 (2014), which is incorporated byreference in its entirety.

Stainless steel cloth electrodes for hydrogen oxidation

The fibrous structure of Pt/C GDEs and favorable interactions betweenthe electrolyte and carbon are responsible for flooding of the electrode(FIG. 11B, FIGS. 17A-17G). The flooding behavior makes Pt/C GDEsunsuitable for high rate gas utilization. Therefore, an alternative GDEsupport was explored to avoid these issues. Stainless steel cloths (SSC)were chosen as the GDE support (FIG. 11C, FIG. 14A), as, unlike carbonfibers, metal threads do not take up electrolyte by capillary action(FIGS. 17A-17G). At non-zero pressures gradients across the SSC, awell-defined separation between the gas and liquid is obtained (FIGS.17A-17G). As stainless steel is a poor hydrogen oxidation catalyst,platinum was electrodeposited (FIGS. 14C-14D and FIGS. 18-22), which isan active HOR catalyst, onto the stainless steel cloths. Theplatinum-coated steel cloths (Pt/SSC) are able to oxidize H₂ in theTHF-based electrolyte with nearly unity FE (FIG. 12A) up to appliedcurrents of 25 mA cm⁻²; higher currents were difficult to test due tolarge electrolyte resistance. This corresponds to approximately a oneorder of magnitude increase in the HOR current when compared to aflooded geometry (FIG. 10B).

Hydrogen oxidation was confirmed by cyclic voltammetry experiments(FIGS. 23A-23E) and by accounting for the mass balance over hydrogen(FIGS. 24A-24B). The increase in the maximum rate of HOR cannot beexplained by a simple electroactive surface area effect, as a highersurface area electrode flooded by electrolyte would be subject to thesame one dimensional transport-limited current density. Breaking thetransport limit and resulting increases in current arise from theestablishment of gas-liquid interfaces which promote gas transport closeto the electrode surface.

The HOR FE demonstrates robustness to changes in non-zero pressuregradients across the Pt/SSC (FIG. 12B). Pressure gradients at or abovethe Laplace pressure of the cloth lead to gas flow into the electrolyte(FIGS. 17A-17G). See, for example, Santamaria, A. D., Das, P. K.,MacDonald, J. C. & Weber, A. Z. Liquid-water interactions withgas-diffusion-layer surfaces. J. Electrochem. Soc. 161, F1184-F1193(2014), which is incorporated by reference in its entirety. While gasflow through the electrode into the electrolyte does not affect the SSCperformance, it may be undesirable for continuous operation due to theneed to control multiple gas streams in the electrochemical cell. Bymaintaining the pressure gradient below the Laplace pressure of thecloth, gas flow exclusively past the electrode can be achieved. Amajority of the experiments described in this work were performed withgas flow past the electrode. At non-zero pressure gradients across thePt/SSC, the flowrate of H₂ past the electrode can be varied with nochange in HOR FE (FIGS. 25A-25D). At the highest applied currents andlowest flowrates, the nominal single pass conversion of hydrogen isapproximately 80% (FIGS. 25A-25D), which is significantly higher thanthe single pass conversion of 25-35% used in traditional Haber-Boschreactors. See, for example, Morgan, E. R., Manwell, J. F. & McGowan, J.G. Sustainable Ammonia Production from U.S. Offshore Wind Farms: ATechno-Economic Review. ACS Sustain. Chem. Eng. 5, 9554-9567 (2017),which is incorporated by reference in its entirety.

In order to demonstrate the generality of the approach of using SSC as aGDE support for nonaqueous solvents, oxidation of hydrogen at high ratesin a 1 M LiBF₄ in 9:1 propylene carbonate/dimethyl carbonate electrolytewas attempted. The electrolyte is similar to electrolytes used in somebatch lithium-mediated nitrogen reduction approaches. See, for example,Kim, K., Chen, Y., Han, J.-I., Yoon, H. C. & Li, W. Lithium-mediatedammonia synthesis from water and nitrogen: a membrane-free approachenabled by an immiscible aqueous/organic hybrid electrolyte system.Green Chem. (2019) doi:10.1039/C9GC01338E; and Kim, K. et al.Electrochemical Synthesis of Ammonia from Water and Nitrogen: ALithium-Mediated Approach Using Lithium-Ion Conducting Glass Ceramics.ChemSusChem 11, 120-124 (2018), each of which is incorporated byreference in its entirety. See, for example, each of which isincorporated by reference in its entirety. It was found that thetransport-limited hydrogen oxidation current density at flooded Pt foilelectrodes is ˜0.25 mA cm⁻² (FIGS. 26A-26D), which is an order ofmagnitude lower than for hydrogen oxidation in THF-based electrolyte(FIG. 10B), likely due to the propylene carbonate electrolyte's higherviscosity and lower hydrogen solubility. By using a Pt/SSC anode, it waspossible to oxidize hydrogen at rates of 25 mA cm⁻² with near unity FE(FIGS. 26A-26D) in the propylene carbonate-based electrolyte, which istwo orders of magnitude higher currents than in the flooded case, evendespite an absence of good proton acceptors in the solution. Thisexperiment demonstrates that SSCs can be used to obtain GDE-likebehavior across different nonaqueous solvents.

Stainless steel cloth electrodes for nitrogen reduction

Having overcome transport limitations for hydrogen oxidation by usingSSCs as an anode, SSCs for the cathodic reaction of nitrogen reductionwas implemented. The lithium metal-mediated approach for N₂ reductionhas been reported to be diffusion limited in THF on flooded copper andsteel foils (FIG. 10C). See, for example, Lazouski, N., Schiffer, Z. J.,Williams, K. & Manthiram, K. Understanding Continuous Lithium-MediatedElectrochemical Nitrogen Reduction. Joule 3, 1127-1139 (2019), which isincorporated by reference in its entirety. In aqueous electrolytes, thetransport limited current density for nitrogen reduction is typicallyeven lower than in nonaqueous systems—approximately 0.5-1.3 mA cm⁻²—anddepends on the hydrodynamics of the electrolyte (see below). Thetransport limited current density can be increased by decreasing theboundary layer thickness through improving the hydrodynamics, forinstance with a rotating disk electrode (RDE) or microfluidic reactor,or by using gas diffusion electrodes. Claims of nitrogen reduction selectrolytes above these rates that do not utilize methods to improvenitrogen transport must be examined with scrutiny adding an additionalcriterion for evaluating the veracity of nitrogen reduction reports.See, for example, Andersen, S. Z. et al. A rigorous electrochemicalammonia synthesis protocol with quantitative isotope measurements.Nature 570, 504-508 (2019); and Greenlee, L. F., Renner, J. N. & Foster,S. L. The Use of Controls for Consistent and Accurate Measurements ofElectrocatalytic Ammonia Synthesis from Dinitrogen. ACS Catal. 8,7820-7827 (2018), each of which is incorporated by reference in itsentirety.

To overcome diffusion limitations, a stainless steel cloth (SSC) wasused as the GDE substrate onto which lithium metal was plated in situ.It was found that the rate of the nitrogen reduction reaction (NRR) issignificantly enhanced compared to the flooded case (FIG. 10C) at highapplied currents (FIG. 12C). This enhancement demonstrates that SSCcathodes can yield rates for continuous NRR that are competitive withliterature reports (FIG. 13A, Table 1). At the highest production rates(FIG. 12C), a total of 8.6±1.4 μmol of ammonia are produced at a 1 cm²electrode after 290 seconds of polarization, resulting in 4.5±0.6 mM and0.45±0.22 mM ammonia concentrations in the electrolyte and trap,respectively. At the highest FEs (FIG. 12D), a total of 11.8±1 μmol ofammonia are produced after 480 seconds of polarization. As the totalamount of charge passed in most experiments is constant and equal to 7.2coulombs, current was applied to the cell for longer periods of timethan 290 seconds in most experiments. In longer duration experiments inwhich more charge is passed, 102 μmol of ammonia were produced over thecourse of 2 hours, with an FE of 18.8% (FIGS. 27A-27B). It was foundthat the total amount of ammonia produced increases monotonically withtime after an induction period of approximately 2 minutes (FIG. 28),which suggests that the lithium-based catalyst cycle (FIG. 10A) rapidlyreaches steady state operation. As the induction period is included inthe time used to compute production rates, the rates reported here areunderestimates of true values for a continuous system. Longer durationexperiments (FIGS. 27A-27B) suggest that the production rate decreaseswith time, which could occur for a number of reasons, including but notlimited to lithium ion depletion, solution phase ammonia affectinglithium plating and nitridation reactions, and incomplete proton donorrecycling. Further experiments are necessary to conclusively determineand address the cause of the decreasing ammonia production rate atlonger times.

Nitrogen reduction control experiments

NH₃ was confirmed to be produced via N₂ reduction by performing controlexperiments in which argon and isotopically labelled N₂ were used asfeed gases (FIGS. 15A-15C). No NH₃ is produced with argon as a feed gas,and there is quantitative agreement between the amount of NH₃ producedwhen ¹⁴N₂ and ¹⁵N₂ are used as feed gases. See, for example, Andersen,S. Z. et al. A rigorous electrochemical ammonia synthesis protocol withquantitative isotope measurements. Nature 570, 504-508 (2019), which isincorporated by reference in its entirety.

The isotope labeling experiments were performed at two differentoperating conditions and architectures, which is necessary forconclusive proof of nitrogen reduction. See, for example, Kibsgaard, J.,Norskov, J. K. & Chorkendorff, I. The Difficulty of ProvingElectrochemical Ammonia Synthesis. ACS Energy Lett. 4, 2986-2988 (2019),which is incorporated by reference in its entirety. The produced NH₃ canbe found in both the solution and the gas phases (FIGS. 12C-12D). Thisis additional evidence of GDE-like behavior of the SSC cathode.Obtaining ammonia in the gas phase is desirable as it may simplifyseparations in a practical process.

In the absence of a pressure gradient across the SSC, the system revertsto a flooded state and generally shows poor efficiency for N₂ reduction(FIG. 12D). While the changes in total NH₃FE with non-zero pressuregradients (FIG. 12D) do not demonstrate a clear trend when accountingfor the measured uncertainty, a maximum FE average combined solution andgas phase FE of 47.5±3.8% was obtained when the pressure gradient was0.5 kPa using a SSC as the cathode. The total NH₃FE also does notsignificantly change when the flowrate of N₂ past the SSC is varied(FIGS. 25A-25D); less NH₃ is found in the gas phase at lower flowrates.The high conversions of N₂ (˜10%) at low N₂ flowrates (FIGS. 25A-25D)are desirable for a practical process; they also are indirect evidenceof N₂ reduction to NH₃, as the fraction of NO_(x) and NH_(x) impuritiesin the stock N₂ would need to be at approximately 1000-10000 parts permillion to yield this much NH₃, several orders of magnitude higher thanthe maximum contamination levels of these impurities reported in thestock gas.

Demonstration of electrochemical Haber-Bosch

After obtaining efficient chemistries using SSCs independently at boththe cathode and anode, the two reactions were coupled into anelectrochemical Haber-Bosch (eHB) reactor, which produces NH₃ from N₂and H₂ at ambient conditions. Using SSC-based GDEs for both electrodesin a single reactor (FIG. 11D), high ammonia yields was maintained atthe cathode (FIG. 13B). At longer reaction times, the advantage of usingH₂ oxidation at a Pt/SSC anode over THF oxidation at a platinum foil isevident, as solvent oxidation is avoided (FIB. 13B). The fate of theproton donor is also addressed by coupling the electrode chemistries.The proton donor, ethanol, is consumed at the cathode to produce NH₃ andethoxide, and could be regenerated from the ethoxide at the anode byprotons produced from H₂ oxidation (FIG. 11D). In long durationexperiments, the number of protons found in ammonia was ˜80% of thenumber of labile protons originally present in ethanol in theelectrolyte. As hydrogen evolution at the cathode also utilizes protonsfrom ethanol, the high utilization of protons suggests that ethanol isindeed regenerated from ethoxide at the anode via hydrogen oxidation.

The eHB reactor operates at ambient conditions, which allows it to beoperated at smaller scales than traditional Haber-Bosch. However, H₂ isusually sourced from steam-methane reforming, which utilizes fossilfuels and is not readily modularized. See, for example, Inc., N.Equipment design and cost estimation for small modular biomass systems,synthesis gas cleanup and oxygen separation equipment. NREL Subcontractreport http://www.nrel.gov/docs/fy06osti/39946.pdf (2006), which isincorporated by reference in its entirety. Water electrolysis is amodular alternative H₂ source. By coupling an electrochemicalHaber-Bosch reactor and a water electrolyzer (FIG. 13C and FIG. 13D),NH₃ was obtained in an overall reaction involving only N₂, H₂O, andrenewable electrons. By using a commercially available water splittingsetup, NH₃ was produced with an FE of 30±2% (FIG. 13B). The slightdecrease in FE compared to eHB is likely from water contamination fromthe H₂ stream. Coupling multiple unit operations in series can beadvantageous for nonaqueous electrochemical ammonia production as it mayincrease the efficiency of each individual step as well as the overallprocess.

While development of GDEs capable of operating in nonaqueous solvents isimperative for practical electrochemical synthesis, many other aspectsof system design require further development. Physical methods torecycle volatile organic solvents and separate the products when usinggaseous feedstocks may be necessary in practical systems. Alternatively,bulk volatile solvents may be replaced with non-volatile analogs withsimilar properties, such as specially tailored polymers or ionicliquids. Polymeric electrolytes may open up avenues for manufacturingspecies-selective membranes for use in nonaqueous systems, analogous toNafion in aqueous systems, and for manufacture of membrane-electrodeassemblies (MEAs) for gas phase reactions. Electrolyte engineering canalso decrease the ionic resistance in the cell, which is important forenergy efficiency at high currents.

In the current work, for instance, the energy efficiency for NH₃production ranges from 1.4 to 2.8% at an applied cell potential of 20-30V, with 70-80% of the energy losses coming from large electrolyteresistance (FIGS. 29A-29B). These values of energy efficiency correspondto energy consumptions of 730-1500 GJ/ton, significantly higher thancontemporary values for Haber-Bosch, or even other lithium-mediatedchemistries. See, for example, McEnaney, J. M. et al. Ammonia synthesisfrom N₂ and H₂O using a lithium cycling electrification strategy atatmospheric pressure. Energy Environ. Sci. 10, 1621-1630 (2017), whichis incorporated by reference in its entirety. However, furtherimprovements to the electrolyte, cell architecture, Faradaic efficiencyand cell lifetime can greatly improve these metrics by reducing sourcesof energy loss (FIGS. 29A-29B).

This work demonstrates the possibility of utilizing metal cloth-basedsupports for high rate electrochemical reactions of sparingly solublegaseous reactants in a nonaqueous solvent. These SSCs were used toproduce ammonia from nitrogen and water-derived hydrogen at the highestreported rates at ambient conditions, 30.4±3.5 nmol cm⁻² s⁻¹(FIG. 13A).The nonaqueous HOR Pt/SSC can find applications in reactions for which acontinuous source of controlled-activity protons is needed, while an SSCat the cathode can be used in producing value-added chemicals fromgaseous feedstocks such as N₂, CO₂, or CO. This approach for utilizinggaseous reactants can become a staple of organic electrosyntheticmethodology.

Methods

Electrolyte preparation

Electrolyte solutions were prepared by dissolving 1 M of LiBF₄(Sigma-Aldrich, 98+%) in molecular sieve-dried tetrahydrofuran (AcrosOrganics, 99+%, stabilized with BHT) to which ethanol (VWR InternationalKoptek, anhydrous, 200 proof) was added to yield an ethanolconcentration of 0.11 M. The obtained solution was centrifuged at 6000rpm for 10 minutes to precipitate insoluble impurities. The clearsolution was transferred to oven-dried glass vials and used within 12hours of preparation. In experiments in which hydrogen oxidation isquantified, ferrocene (Alfa Aesar, 99%) is added to the solutions toyield a ferrocene concentration of ˜0.25 M.

Preparation of platinum-coated steel cloths

Stainless steel cloths (McMaster-Carr, 304 stainless steel, 400×400mesh) were electroplated with nickel followed by platinum (FIGS.18A-18B). A Wood's nickel strike solution, which consists of 1 M NiCl₂(Sigma-Aldrich) and 1 M HCl (Sigma-Aldrich) in water, was used to platenickel onto the cloths (FIGS. 19A-19F). The cloth was used as theworking electrode while a piece of nickel foil (Alfa Aesar, 99+%) wasused as the soluble counter electrode in an undivided beaker cell. Thecloth was pretreated by applying an oxidative current of 15 mA cm_(geom)⁻² for 30 seconds, immediately after which a reductive current of 30 mAcm_(geom) ⁻² was applied for 5 minutes obtain a nickel-plated stainlesssteel cloth (FIGS. 20A-20F).

After striking the cloth with nickel, platinum can be deposited. Theplatinum plating solution used was a citrate-ammonium bath, containing35 mM (NH₄)₂PtCl₆ (Alfa Aesar), 400 mM trisodium citrate (anhydrous,Beantown Chemical), and 75 mM of NH₄Cl (Alfa Aesar). The nickel-strickencloth was used as the working electrode; a piece of platinum foil wasused as a soluble counter electrode in a beaker cell, which was keptover a water bath at 90° C. A constant reductive current of 10 mA (˜5 mAcm_(geom) ⁻²) was applied to the cloth for 5 minutes. Theplatinum-coated cloths (FIGS. 21-22) were thoroughly rinsed with DIwater to remove any ammonium containing compounds from the surface anddried at 80° C. in air prior to use.

Gas diffusion electrode experiments

Experiments were performed in 3-compartment cells (FIGS. 30-31), inwhich working and counter electrode compartments were separated by aDaramic 175 separator; all cell parts were oven-dried at 80° C. beforeuse. The working electrode was a piece of stainless steel cloth innitrogen reduction experiments or a piece of platinum-coated stainlesssteel cloth in hydrogen oxidation experiments. The working electrode wasfixed between the working electrode compartment and a gas compartment.The working gas (e.g. N₂ or H₂) was flowed first through a vialcontaining THF to saturate the gas with THF, followed by the gascompartment of the electrochemical cell; the gas was slightlypressurized by a water column at the outlet of the gas compartment(FIGS. 32A-32D). In experiments where propylene carbonate-basedelectrolyte was used, the vial contained propylene carbonate instead ofTHF.

1.75 mL of electrolyte was added to each electrode compartment, for atotal electrolyte volume of 3.5 mL. Note that this is the volume ofelectrolyte added to each compartment and may not be the finalelectrolyte volume due to electrolyte evaporation. At this point, apressure gradient was established across the working electrode due tothe fact that the electrolyte compartment is at atmospheric pressurewhile the gas compartment is at positive pressure due to the watercolumn at the compartment outlet; this pressure gradient preventselectrolyte flow into the gas compartment and establishes a robustgas-electrolyte interface. Initially, the height of the water column waschosen to redirect gas flow into the electrolyte compartment. Gas wasflowed into the electrolyte for 10 minutes at 10 standard cubiccentimeters per minute (sccm) to saturate the solution with the gas. Theheight of the water column was then lowered to obtain the desiredpressure gradient across the SSC while maintaining flow past the SSC. Incertain experiments, the flowrate of the gas past the electrode was alsodecreased at this stage (FIGS. 25A-25D). A constant current was thenapplied to the cell; in most experiments, 7.2 C of charge were appliedirrespective of current density.

For nitrogen reduction experiments, an additional vial containing 2 mLof 0.1 M H₃BO₃ (Alfa Aesar, 99.99%) was added between the gascompartment and the water column to capture any gas phase ammonia (FIGS.32A-32D). To quantify ammonia, the catholyte was diluted to 100 mL in avolumetric flask in water, after which the obtained solutions wereeither used as-is or diluted 2- or 4-fold further for ammoniaquantification via a colorimetric assay (FIG. 33). The boric acid trapwas quenched with 500 μL of 0.4 M NaOH before being diluted to 25 mL ina volumetric flask. The ammonia content in the trap-derived solution wasquantified via a colorimetric assay without further dilution.

For some hydrogen oxidation experiments, the electrolyte containedferrocene at a concentration of ˜0.25 M, the oxidation of which was usedto estimate the FE toward hydrogen oxidation (see SupplementaryDiscussion). In mass-balance closure and eHB cell experiments, theelectrolyte was unchanged from the one used in nitrogen reductionexperiments.

Nitrogen reduction control experiments

Nitrogen reduction to ammonia at SSC cathodes was confirmed by varyingthe feed gas in NRR experiments (FIGS. 15A-15C). All feed gases passedthrough three solutions before entering the cell to purify and preparethe gas. First, the gases were passed through 0.1 M NaOH to capture anyNO_(x) in the gases, then through 0.05 M H₂SO₄ to capture any NH₃,followed by THF with sieves to capture water in the gaseous stream andto saturate the gas with THF. Initially, 10 sccm of argon gas werepassed through all three solutions to remove air, which contains ¹⁴N₂and O₂. Then, 5 sccm of the desired gas —Ar (Airgas), ¹⁴N₂ (Airgas), or¹⁵N₂ (Cambridge Isotope Laboratories)—was passed through the solutionsand fed to the cell for nitrogen reduction experiments.

Two architectures were used to confirm nitrogen reduction. In one set ofexperiments, a 3-compartment cell (FIGS. 30A-30B) with a SSC cathode, Ptfoil anode, and a Daramic separator was used; 25 mA of current wasapplied to the cell for 290 seconds in these experiments. In the secondset of experiments, a 4-compartment cell (FIG. 12D) with a SSC cathode,Pt/SSC anode, and no separator was used; in this configuration, 5 sccmof H₂ (Airgas) was fed to the anode. 20 mA of current was applied for360 seconds in these experiments.

The catholyte from the 3-compartment cell experiments, all theelectrolyte from the 4-compartment cell experiments, and the acidtrapped were acidified with 0.05 M H₂SO₄ in water to convert all NH₃ toNH₄₊. NMR spectra of the obtained solutions were taken with solventsuppression on a Bruker Avance Neo 500.18 MHz spectrometer (FIG. 15C).The ammonia content of the solutions was quantified via the colorimetricassay; the measured concentrations of ammonia were found to beconsistent with those measured by NMR (FIGS. 35A-35C).

Hydrogen oxidation quantification

To quantify the HOR FE, an excess of ferrocene (˜0.25 M) was added tothe electrolyte prior to electrolysis. As ferrocene is thermodynamicallymore difficult to oxidize than H₂, but easier than THF (FIG. 10B), anyapplied current would first oxidize H₂, assuming the oxidation iskinetically facile, followed by ferrocene, once HOR is diffusionlimited. After application of current in HOR experiments using Pt/SSCanodes, the anolyte was diluted to 10 mL with N₂-purged water. Theproduced cloudy orange mixture was extracted with N₂-purged hexanesthree times. The obtained solution was centrifuged for improved phaseseparation. The ferrocenium content of the produced clear aqueoussolutions was quantified via UV-vis spectroscopy by using a combinationof the visible (619 nm) and UV (255 nm) ferrocenium absorption peaks(FIGS. 36A-36D). The concentration of ferrocenium in solution was thenused to estimate the maximum value of HOR FE. For a detailed discussionof the method, see below.

For experiments at low flowrates and high currents (i.e. at highconversions), the HOR FE was also computed by estimating the hydrogenflowrate out of the gas compartment and by using a hydrogen mass balanceover the gas compartment (FIGS. 24 and 26). The results were consistentwith those obtained via the ferrocene-based method. For a detaileddiscussion of the method, see below.

Other examples follow. The procedures and materials for nitrogenreduction outlined below have been optimized for reliable ammoniaproduction. However, certain deviations from the procedure and materialvendors, which are specifically called out in-text and in prior work,may lead to poorer ammonia production. See, for example, Lazouski, N.,Schiffer, Z. J., Williams, K. & Manthiram, K. Understanding ContinuousLithium-Mediated Electrochemical Nitrogen Reduction. Joule 3, 1127-1139(2019), which is incorporated by reference in its entirety. Follow theprocedure closely for reproducibility and high yields.

Electrolyte solution preparation

Dry THF was used as the solvent in THF-based experiments describedbelow. It was obtained by drying as-purchased THF over 20% v/v offreshly dried molecular sieves for at least 48 hours in a round-bottomflask sealed with a rubber septum stopper. The sieves were prepared bywashing with acetone and heating at 300° C. for 5 hours in a mufflefurnace. The water content of dry THF was found to be 7.1±0.3 ppm (n=3)via Karl-Fischer titration. As-purchased LiBF₄, stored in an Arglovebox, was dissolved in dry THF to obtain electrolyte solutionscontaining 1 M LiBF₄. As discussed in Lazouski et al., it is imperativefor the LiBF₄ to be pure; LiBF₄ purchased from Sigma-Aldrich was foundto be sufficiently pure for these experiments, while other vendors' mayrequire purification. Ethanol was added to the solution to obtain aconcentration of EtOH of 0.11 M. Insoluble residue was removed from thesolutions by centrifugation at 6000 rpm (4430 rcf) for 10 minutes. Clearelectrolyte solutions were stored in oven-dried glass vials in adesiccator and used within 12 hours of preparation. While the solutionsare somewhat water sensitive, performing centrifugation and solutiontransfer operations in atmospheric air is permissible, as long as theoperations do not expose the solutions to air for long periods of time(hours, i.e. during storage). Oxygen from the atmosphere is typicallypurged from the electrolyte during saturation by gas flow (see below).

In propylene carbonate-based experiments, solvents and the electrolytesalt were used as received. LiBF₄ was dissolved in a 9:1 by volumemixture of propylene carbonate and dimethyl carbonate to produce a 1 MLiBF₄ in PC/DMC electrolyte. Dimethyl carbonate was added to theelectrolyte in order to reduce the viscosity of the solution, as opposedto using pure propylene carbonate as the solvent. The resulting solutionwas centrifuged at 6000 rpm (4430 rcf) for 20 minutes to removeinsoluble impurities. Clear electrolyte was transferred to oven-driedvials and used within 12 hours of preparation.

Nitrogen reduction experiments—steel foils

Flooded steel foil experiments (FIG. 10C) were performed analogously tocopper foil experiments described in Lazouski et al. Briefly,two-compartment cells were assembled with a steel foil cathode, platinumfoil anode, and a Daramic separator. Steel foils were prepared bypolishing with 400 grit sandpaper, followed by 1500 grit sandpaper, andfinally by rinsing with DI water and drying in air at 80° C. Daramicseparators were prepared by successively soaking them in THF and waterseveral times, as follows: as-received separators were immersed inas-purchased THF for ˜10 minutes with agitation, after which thesolution turned yellow. The separators were then immersed in water for˜10 minutes to remove THF and water-soluble impurities. The process ofimmersing in THF and water was repeated two more times, after which theseparators were dried in air at 80° C. On rare occasions, fresh Daramicseparators may initially lead to poor ammonia yields, likely due tocontaminants left in the porous structure. Ammonia yields reach steadyvalues after reusing the separator twice or more. All cell parts,separators, and electrodes were washed with DI water and dried in anoven at 80° C. prior to use. Daramic separators are also soaked in DIwater for at least 10 minutes to remove electrolyte and residual ammoniafrom the structure prior to drying. It is recommended to rinse the cellparts with THF, acetone, and water if cell parts are new or have beenpreviously used for other chemistries to remove impurities that maynegatively affect yields. Cell parts and separators, and platinum foilsare reused multiple times, while the steel foils are used once perexperiment. 1.75 mL of 1 M LiBF₄, 0.11 M EtOH in THF electrolyte wasadded to each compartment of the cell. Note that this is the volume ofelectrolyte added to each compartment and may not be the finalelectrolyte volume due to electrolyte evaporation. THF-saturated N₂ gaswas bubbled through the cathode compartment at 10 standard cubiccentimeters per minute (sccm) for 10 minutes to saturate the solutionwith N₂. A chosen constant current was then supplied to the cell using aBiologic VMP3 potentiostat, Tekpower TP3005T or Tekpower TP5003T DCpower supply. When using a power supply, the current was monitored bymeasuring the potential drop across a calibrated 33-Ohm resistor inseries with the electrochemical cell (FIGS. 38A-38D). Approximately 7.2coulombs of charge were supplied in each experiment. The catholyte wasremoved from the cell and diluted with Milli-Q water in a 50 mLvolumetric flask. The cathode compartment was rinsed twice with Milli-Qwater and added to the flask, after which the solution was diluted tothe mark. Three samples for ammonia quantification were made: onecontaining only 2000 μL of the diluted catholyte, one containing 1000 μLof the diluted catholyte and 1000 μL of Milli-Q water, and onecontaining 500 μL of the diluted catholyte and 1500 μL of Milli-Q water.

Assembly of gas diffusion cell

Gas diffusion experiments were performed using 3- and 4-compartmentcells (FIGS. 30A-30B, FIG. 13D). In a 3-compartment cell, there are gas,working, and counter compartments, the latter two separated by a pieceof Daramic. Daramic separators were prepared by washing in THF andwater, as described above in Nitrogen reduction experiments—steel foils.

All cell parts, separators, and electrodes were dried in an oven at 80°C. prior to use. Aluminum current collectors for the GDE were made from0.016″ thick 6061 aluminum; the hole was made with a 7/16″ wood drillbit and sanded to smooth the edges. O-rings between the compartments(FIGS. 30 and 31) are important for making a good gas seal.Fluoropolymer-coated silicon O-rings are recommended, as they arecompatible with THF. The procedure to assemble a cell can be seen inFIGS. 31A-31H. In words, a “stack” is made by placing the pieces in thefollowing order:

Once the cell is assembled, the inlet of the gas compartment is attachedto a bubbler containing THF, through which the gas is bubbled. Theoutlet is attached to a tube that enters a tall (50 cm) burettecontaining a water column to control the gauge pressure inside the gascompartment and the pressure gradient across the GDE (FIGS. 32A-32D); inNRR experiments, a 0.1 M H₃BO₃ in Milli-Q acid trap is inserted betweenthe outlet of the cell and the burette (FIGS. 32A-32D) to capture gasphase ammonia.

After the cell is set up and gas is flowing through the bubbler at 10sccm, 1.75 mL of LiBF₄/EtOH/THF electrolyte is added to the countercompartment, followed by 1.75 mL of electrolyte to the workingcompartment. Note that this is the volume of electrolyte added to eachcompartment and may not be the final electrolyte volume due toelectrolyte evaporation. Initially, some of the electrolyte in theworking compartment may enter the gas compartment; however, after theentire GDE is contacted by the electrolyte, the pressure in the gascompartment increases, as evidenced by motion of the gas level in theburette, and the electrolyte returns to the working compartment, afterwhich gas begins to flow through the GDE (FIGS. 17A-17G). When usingbare steel cloths, the pressure in the gas compartment is typically1.6-1.8 kPag, corresponding to 16-18 centimeters of water column abovethe gas. The pressure in a flow through configuration is 1.8-2.0 kPagwhen platinum-coated steel cloths are used. The working compartmentelectrolyte is saturated with the fed gas by flowing it through the GDEfor 10 minutes. Following saturation, the water level in the burette isdecreased to lower the pressure in the gas compartment to the desiredvalue and favor gas flow past the GDE and out of the gas compartment(FIGS. 32A-32D). Some gas bubbles begin to flow out into the burettealmost immediately upon lowering of the pressure in the gas compartmentbelow the flow-through pressure. The addition of an acid trap in NRRexperiments (FIGS. 32A-32D) adds approximately 2 cm of water ofadditional gauge pressure in the gas compartment, which is accounted forwhen computing pressure gradients across the GDE. At this point, theflowrate of the gas can be reduced if desired, such as in experimentswhere the effect of flowrate past the electrode is quantified, afterwhich current is applied. The electrolyte in the working compartment wasremoved and analyzed after application of current.

In a 4-compartment cell used in combined electrochemical Haber-Bosch(eHB) experiments, the setup is similar to the aforementioned3-compartment cell. The main change is that the counter electrode isreplaced by a second gas compartment and gas diffusion electrode. Insummary, the stack becomes:

Nitrogen reduction experiments—steel cloth experiments

Nitrogen reduction reaction (NRR) experiments focused on studying theNRR on steel cloths were performed in 3-compartment cells, the setup ofwhich is detailed in Assembly of a gas diffusion cell. A platinum foilanode and a circular stainless steel cloth (SSC) cathode (diameter=14mm) were used as the electrodes. The steel cloth electrode was rinsedwith DI water and dried at 80° C. prior to use; it was used only for oneexperiment before discarding. The platinum foil was reused indefinitely.The method of applying current and preparing quantification samples wasanalogous to the method described in Nitrogen reductionexperiments—steel foil experiments, with one change: the catholyte wasdiluted in a 100 mL volumetric flask due to higher amounts of producedNH₃.

One modification that was made in NRR experiments was the addition of a2 mL 0.1 M boric acid trap between the gas compartment and thepressure-controlling burette to capture any gas phase ammonia. Inexperiments where gas was fed through the SSC, the trap was inserted atthe gas outlet in the catholyte chamber, so all fed gas still wentthrough it. When no pressure gradient across the SSC was applied (FIG.12D), no boric acid trap was present, hence no quantification of gasphase products could be performed. While this may have underestimatedthe amount of ammonia produced, it is likely that little ammonia wasfound in the gas phase in this experiment, as ammonia stripping frombulk solution does not occur (FIGS. 17A-17G and FIGS. 39A-39D). Aftereach experiment, the solution in the trap was quenched with 500 μL of0.4 M NaOH and diluted in a 25 mL volumetric flask with Milli-Q water,after which the ammonia concentration in the solution was quantified.

Nitrogen reduction experiments—time evolution of ammonia

Experiments were performed in which the concentration of ammonia in theelectrolyte was measured as a function of operating time (FIG. 28). Inthese experiments, the setup was analogous to the experiments describedin Nitrogen reduction experiments—steel cloth experiments, except that1.9 mL of electrolyte was added to the cathode compartment instead ofthe usual 1.75 mL, providing extra electrolyte for periodically removingaliquots. A 20 mA cm⁻² current was applied for 10 minutes in theseexperiments to maintain a high production rate while allowing for goodtemporal resolution of the data. 10 sccm of THF-saturated N₂ was flowedthrough the SSC to allow for bulk mixing in the electrolyte. Every 60seconds, nominally 20 μL of catholyte was extracted from the cathodecompartment via pipette. Because the electrolyte forms a thin film onthe inside of the pipette tips that cannot readily be utilized, precisequantification of the amount withdrawn is difficult. By measuring themass of electrolyte that can be recovered from the tips (not during theexperiment), it was found that approximately 16 μL of electrolyte areused to make quantification samples. The extracted catholyte was dilutedin 2 mL of Milli-Q water. The ammonia content of the two samples wasquantified as described below. The amount of ammonia produced at eachtime point (FIG. 28) was computed according to Equation 1.

n _(NH) ₃ (t)=(V ₀−0.02·t)·C(t)  #(1)

After the experiment, the cathode was diluted in a 100 mL volumetricflask to quantify the remaining ammonia.

Ammonia quantification—calibration solution preparation A freshcalibration curve was made during each batch of quantifications. Thecalibration solutions contained a known amount of NH₄Cl in Milli-Q water(FIGS. 34A-34D). While ammonia samples from NRR experiments had someamount of LiBF₄, EtOH, and THF in them, they were sufficiently dilute asto not affect the quantification significantly (FIGS. 34A-34D). Theabsorbance of solutions containing THF is diminished, potentiallyleading to an underestimate in ammonia concentration, as pure watercalibrations curves (FIGS. 34A-34D) were used for all analyses.

¹⁵N₂ isotope labeling experiments

Isotope labeling experiments were used as a control to confirm N₂reduction to NH₃. In order to remove any NH₃ and NO_(x) potentiallyfound in ¹⁵N₂ stock (and house ¹⁴N₂), the gases were successively passedthrough solutions of 0.1 M NaOH in water (to capture NO_(x)), 0.05 MH₂SO₄ in water (to capture NH₃), and THF containing activated molecularsieves (to capture water and to saturate the gas with THF) before beingfed to the cell. Two isotope labeling experiments were performed: oneutilized a typical 3-compartment cells with a steel cloth cathode,Daramic separator, and platinum foil anode with pressure control, whilethe other utilized an eHB reactor with a steel cloth cathode, Pt/SSCanode, and no separator.

Prior to any experiments, 10 sccm of Ar were bubbled through the entiresetup (traps, bubbler, and cell) for 15 minutes to remove trace amountsof ¹⁴N₂ and other impurities. The desired gas (Ar, ¹⁴N₂, or ¹⁵N₂) wasfed to the cell and through the SSC at 5 sccm for 10 minutes. In the eHBexperiment, H₂ was fed through the Pt/SSC at 5 sccm for 10 minutes aswell.

In the 3-compartment cell experiment, the pressure gradient across theSSC was decreased to 1 kPa, after which 25 mA of current was applied for4.8 minutes. The catholyte was removed from the cell into a glass vial.The cathode compartment was rinsed with 0.05 M H₂SO₄ twice and theresulting fractions were added to the catholyte. The entire mixture wasdiluted to ˜4 mL with 0.05 M H₂SO₄. Similarly, the boric acid trap wasdiluted to a total volume of ˜4 mL with 0.05 M H₂SO₄. The solution wasacidified to convert all the ammonia to ammonium (NH₄ ⁺) for NMRanalysis. See, for example, Nielander, A. C. et al. A Versatile Methodfor Ammonia Detection in a Range of Relevant Electrolytes via DirectNuclear Magnetic Resonance Techniques. ACS Catal. 9, 5797-5802 (2019),which is incorporated by reference in its entirety.

In the eHB experiment, the pressure gradient across both electrodes wasdecreased to 1 kPa, after which the gases were flowed past theelectrodes for an additional 2 minutes. 20 mA of current were passed for6 minutes. The electrolyte was removed from the cell into a 10 mLvolumetric flask. The cell was rinsed with 0.05 M H₂SO₄ and theresulting solutions were added to the volumetric flask and diluted tothe mark (10 mL). The boric acid trap was diluted to a total volume of˜4 mL.

The ammonia content in the resulting solutions was quantified using thesalicylate method described above. The electrolyte solutions forquantification were neutralized and diluted 50-fold (40 μL of sample, 10μL of 0.4 M NaOH, 1950 μL of water) in 3 compartment case or 20-fold(100 μL of sample, 25 μL of 0.4 M NaOH, 1875 μL of water) in the eHBcase, while the trap solutions were diluted 10-fold (200 μL of sample,50 μL of 0.4 M NaOH, 1750 μL of water). NMR spectra of the undilutedsolutions were measured on a three-channel Bruker Avance Neo 500.18 MHzspectrometer. Solvent suppression of the largest one (H₂O) or three(THF+H₂O) peaks was used to increase the signal-to-noise ratio forammonium peaks. Locking and shimming was done on ¹H from water in thesolution; no additional compounds were added to the solution prior toNMR. 64 scans were measured for all spectra. The N—H coupling in the NMRspectra confirms ammonia formation from feed N₂ (FIG. 15C).

Preparation of platinum-coated steel cloths

In order to prepare an SSC for hydrogen oxidation, platinum metal, aneffective HOR catalyst, was electrodeposited onto the steel cloths. Itwas found that platinum metal has poor adhesion to stainless steel,which has also been observed in the literature. See, for example,Stoychev, D., Papoutsis, A., Kelaidopoulou, A., Kokkinidis, G. &Milchev, A. Electrodeposition of platinum on metallic and nonmetallicsubstrates—selection of experimental conditions. Mater. Chem. Phys. 72,360-365 (2001), which is incorporated by reference in its entirety. Inview of this, the steel cloths were first treated by “striking” withnickel. A Wood's nickel strike solution, which consists of 1 M NiCl₂ and1 M HCl in water, was used. Typically, a piece of steel cloth that is 3cm by 5 cm is taken and submerged to have 2.5 cm by 5 cm in the nickelstrike solution. The cloth was used as the working electrode while apiece of nickel foil was used as the soluble counter electrode in anundivided beaker cell. The cloth was pretreated by applying an oxidativecurrent of 15 mA cm_(geom) ⁻² for 30 seconds, immediately after which areductive current of 30 mA cm_(geom) ⁻² was applied for 5 minutes obtaina nickel-plated stainless steel cloth. The cloth was thoroughly rinsedwith DI water and dried in air at 80° C. The cell potential required fornickel plating was typically ˜1 V. Some of the current went towardhydrogen evolution, evidenced by gas evolution on the cloth; assuming90% FE toward nickel plating, the resulting nickel layer isapproximately 3 μm thick. The cloths visibly change colors afterstriking with nickel (FIGS. 18A-18B).

After striking the cloth with nickel, platinum can be deposited. Thenickel-stricken cloth was cut into smaller pieces to submerge ˜1.5 cm by1.5 cm into 10 mL of a platinum plating solution. The platinum platingsolution used was a citrate-ammonium bath, chosen for its high currentefficiency toward platinum plating, low current density required, andthe non-hygroscopic nature of the platinum precursor. See, for example,Rao, C. R. K. & Trivedi, D. C. Chemical and electrochemical depositionsof platinum group metals and their applications. Coord. Chem. Rev. 249,613-631 (2005); and Baumgartner & Raub. The Electrodeposition ofPlatinum and Platinum Alloys. Platin. Met. Rev. 32, 188-197 (1988), eachof which is incorporated by reference in its entirety. The bathcontained 35 mM (NH₄)₂PtCl₆, 400 mM trisodium citrate, and 75 mM ofNH₄Cl. The nickel-stricken cloth is used as the working electrode; apiece of platinum foil is used as a soluble counter electrode in abeaker cell, which is kept over a water bath at 90° C. It is possiblethat the (NH₄)₂PtCl₆ will not fully dissolve until the solution reaches90° C. A constant reductive current of 10 mA (˜5 mA cm_(geom) ⁻²) wasapplied to the cloth for 5 minutes. The cell potential required forplatinum plating is typically ˜1.7-1.8 V. If the potential was lowerthan ˜1.7 V, then a higher current, up to 20 mA, was applied; this iscommon for fresh baths. The cloth should turn darker after platinumplating (FIGS. 18A-18B). The cloth was thoroughly rinsed with DI waterand dried at 80° C. A 14 mm diameter circle was cut out to be used asthe HOR electrode, and was typically used only once per experiment. Theentire platinum deposition procedure is unoptimized and may requirefurther improvement for practical applications.

Ammonia contamination in the cathode compartment from the Pt/SSC anodeis unlikely as using Pt foils (in 3-compartment experiments as opposedto Pt/SSC in 4-compartment and undivided experiments) which have notbeen in contact with the ammonium bath as the anode also leads toproduction of ammonia in NRR experiments (FIG. 13B). Ar and isotopelabeling experiments confirm ammonia production via nitrogen reduction(FIGS. 15A-15C).

Hydrogen oxidation experiments

Hydrogen oxidation experiments focused on studying HOR were performed in3-compartment cells, the setup of which is detailed in Assembly of a gasdiffusion cell. The cell parts and separators were washed with acetone(to remove residual ferrocene) and water and dried in an oven at 80° C.between experiments. A steel foil was used as the cathode, whilePt-coated steel cloths or Pt/C carbon paper disks were used as theanode; both the anode and cathode materials were fresh in everyexperiment and used only once. 10 sccm of H₂ was flowed past the anodeafter saturating the electrolyte with H₂. Regardless of current applied,7.2 coloumbs of charge were passed though the cell, after which hydrogenoxidation Faradaic efficiency was quantified as described below inQuantification of hydrogen oxidation. In certain experiments (FIG. 12B),no pressure control in the gas compartment was used: the hydrogen gaswas vented directly to a continuously operating fume hood. Some controlexperiments were performed where N₂ gas was fed instead of H₂ gas to theanode; the HOR FE was found to be zero to within error (FIGS. 36A-36D).

Quantification of hydrogen oxidation via mass balance

In cases where the flowrate of gas past GDEs is low and the appliedcurrent is high, the conversion of the gas may be high (FIGS. 25A-25D).The nominal conversion 4 in this case is defined in Equation 3.

$\begin{matrix}{\xi = \frac{I_{applied} \cdot {FE}}{n\;{F \cdot Q_{applied} \cdot \frac{P_{0}}{{RT}_{0}}}}} & {\#(3)}\end{matrix}$

In Equation 3, I_(applied) is the applied current density, FE is theFaradaic efficiency towards the reaction of interest, n is the number ofelectrons involved in the electrochemical reaction (n=2 for HOR, n=6 forNRR), F is Faraday's constant, Q_(applied) is the flowrate of gas setusing the flow controller, R is the universal gas constant, T₀ and P₀are the standard temperature and pressure (273K, 1 bar), respectively,used if the Q_(applied) is given in units standard volumetric flow units(e.g. sccm).

The conversion can be estimated by measuring the outlet flowrate of gasand by using a mass balance over the gas in the gas compartment, givenin Equation 4.

Q _(in) −Q _(out) =ξQ _(in)  #(4)

From Equations 3 and 4, the Faradaic efficiency of gas conversion can becomputed. Implicitly, one can assume that the gas compartment does nothave leaks, and that gas dissolution into the electrolyte is negligible.Practically, this process may be used to quantify hydrogen oxidation, asthe conversions can be very high.

The flowrate of gas leaving the compartment is difficult to quantifyusing a flow controller as it is directed to a pressure-controllingwater column which releases the gas to the ambient environment and addsadditional back pressure which may be difficult to control (FIGS.32A-32D). Therefore, to quantify the flowrate of gas leaving the gascompartment, the time between bubbles of gas detaching the tubing in thewater column (FIGS. 32A-32D) was measured by an electronic stopwatch.This method assumes that the volume of gas bubbles is fairly constantand independent of flowrate, i.e. that they only detach when they reacha critical volume. The critical bubble volume was found to change withtubing orientation and gas, and so only results in the same set can becompared directly. The bubble size can be “calibrated” by measuring theinterval between bubble detachments at a given flowrate when no currentis applied. Thus, the outlet flowrate was computed using Equation 5.

$\begin{matrix}{Q_{out} = \frac{Q_{{nominal}\mspace{14mu}{in}} \cdot t_{{no}\mspace{14mu}{current}}}{t_{{with}\mspace{14mu}{current}}}} & {\#(5)}\end{matrix}$

The HOR FE quantification experiments were performed using 3-compartmentcells with a Pt/SSC anode, Daramic separator, and steel cathode. Theelectrolyte used was either 1 M LiBF₄/0.11 M EtOH/THF (FIGS. 24A-24B) or1 M LiBF₄ in 9:1 PC/DMC (FIGS. 26A-26D). First, the average time forbubble detachment was measured when flowing 0.5 sccm of the desired gas(N₂ for controls and H₂ for HOR quantification) without applyingcurrent. Then, 25 mA of current was applied without changing tubingconfiguration or flowrate, and the average time for bubble detachmentwas recorded.

It was found that when N₂ is fed to the cell, the interval betweenbubbles does not significantly change for either tested electrolyte whencurrent is applied (FIGS. 24A-24B and FIGS. 26A-26D), while the intervalincreases when H₂ is fed and current is applied (FIGS. 24A-24B). TheFaradaic efficiency for HOR can be estimated from these data; it wasfound to be 105±2% for oxidation using THF-based electrolyte (FIGS.24A-24B and FIG. 26) and 112±19% for PC-based electrolyte (FIGS.26A-26D).

A limitation with the method of mass balancing was found, however. Whenhydrogen is fed a rate of 0.2 sccm to a 3-compartment cell with a Pt/SSCanode and THF-based electrolyte and 25 mA of current are applied, thereshould be some gas leaving the gas compartment, as 25 mA corresponds toa hydrogen oxidation rate of at most 0.176 sccm at 100% FE. However, itwas observed that no bubbles are evolved and the level of the totalamount of gas decreases with time; a constant amount of gas with 25 mAof applied current is obtained when H₂ is fed at a rate of 0.22-0.23sccm. While this demonstrates the limitation of the mass balance methodand possible unaccounted for sources of hydrogen depletion, hydrogen isoxidized at high rates in this system with close to unity FE.

eHB experiments and coupling to water splitting

A 4-compartment cell with a steel cloth cathode and Pt/SSC anode wasassembled as described in Assembly of a gas diffusion cell. Theoperation of the 4-compartment cell was similar to operation of the3-comparment cells in NRR and HOR experiments. 3.5 mL of 1 M LiBF₄, 0.11M EtOH in THF was added to the 4-compartment cell (1.75 mL to eachcompartment), while 10 sccm of THF-rich N₂ and 10 sccm of THF-rich H₂were fed to the cathode and anode compartments, respectively. Thesolutions were saturated with their respective gases for 10 minutes byflowing gas through the SSCs. The pressure gradient across the SSCs waslowered using a water column to 1 kPa, at which point the gas flowedpast the SSC. 25 mA were applied to the cell for 4.8 minutes, afterwhich the ammonia content of the cathode chamber was analyzed asdescribed in Nitrogen reduction experiments.

In long term experiments (FIG. 13B), 20 mA were applied for 1 hour in a3-compartment cell with a platinum foil anode or 1-2 hours in a4-compartment eHB cell. Photographs of the anolyte solutions were taken.In long duration experiments to assess the efficacy of nitrogenreduction, eHB reactors were used. In certain experiments (FIGS.27A-27B), the Daramic was replaced with a thinner Celgard separator topromote diffusion between the electrolyte compartments, or the separatorwas removed completely. 20 mA of current were applied to the reactor,while 5 sccm of N₂ and H₂ flowed past the cathode and anode GDEs,respectively.

To couple the eHB to water splitting, a commercially availablewater-splitting cell (Fuel Cell Technologies) was assembled. Theelectrodes were part of a membrane electrode assembly (MEA) purchasedfrom FuelCellStore with an electrode area of 5 cm². The cathode side wasplatinum black with a loading of 3 mg cm⁻²; the anode side was iridiumruthenium oxide with a loading of 3 mg cm⁻²; both electrodes were on aNafion 115 membrane. The bolts on the electrolyzer were tightened with atorque wrench with a torque of 40 lb-in.

Milli-Q water was fed continuously to the anode of the water splittingcell at ˜70 mL/min with a peristaltic pump. A constant current of 200 mAwas applied across the electrolyzer; the voltage required was 1.59 V.This corresponds to an output H₂ flowrate of 1.5 sccm. The cell wasslightly angled to help oxygen bubbles to leave the anode compartment.The cathode compartment was sealed off at one end to force hydrogen toflow in a single direction. The hydrogen was first fed to a vialcontaining magnesium sulfate (MgSO₄) to capture some of the moisture inthe gas stream, after which it was fed to a vial with THF and molecularsieves to saturate the gas with THF. The cell was then operatedanalogously to the way an eHB cell was, with the difference that thefeed rate of H₂ was 1.5 sccm, as defined by the water splitting current.

DISCUSSION

Computing the diffusion-limited current density for H₂ oxidation

One can estimate the diffusion-limited current density for H₂ oxidationand find that it is fairly close to the value one can obtain via directmeasurement (FIG. 10B). The diffusion-limited current density can becomputed via Equation 6. See, for example, Bard, A. J. & Faulkner, L. R.Electrochemical Methods. Fundamentals and Applications. (John Wiley &Sons, Inc, 2001), which is incorporated by reference in its entirety.

$\begin{matrix}{J_{\lim} = \frac{2D_{H_{2}}C_{H_{2}}F}{\delta}} & {\#(6)}\end{matrix}$

The solubility of hydrogen in pure THF is 3.3-3.4 mM.^(9,10) See, forexample, Gibanel, F., López, M. C., Royo, F. M., Santafé, J. & Urieta,J. S. Solubility of nonpolar gases in tetrahydrofuran at 0 to 30° C. and101.33 kPa partial pressure of gas. J. Solution Chem. 22, 211-217(1993); and Brunner, E. Solubility of Hydrogen in 10 Organic Solvents at298.15, 323.15, and 373.15 K. J. Chem. Eng. Data 30, 269-273 (1985),each of which is incorporated by reference in its entirety. However, theproperties of the solvent change with addition of large amounts ofelectrolyte, leading to a “salting-out” effect, decreasing thesolubility of the gas. See, for example, Weisenberger, S. & Schumpe, A.Estimation of Gas Solubilities in Salt Solutions at Temperatures from273 K to 363 K. AIChE J. 42, 298-300 (1996), which is incorporated byreference in its entirety. One can estimate that the solubility ofhydrogen in the electrolyte is close to half of its pure solventsolubility, approximately 1.7±0.8 mM, following Lazouski et al. Thediffusivity of hydrogen in the electrolyte is also an estimate, computedby using an approximate value of the viscosity of solution, and assumedto be 3.8±0.8-10⁻⁹ m² s⁻¹. The diffusion boundary layer thickness waspreviously measured and found to be 50±15 μm;¹ corrections to thediffusion boundary layer thickness due to differences in diffusioncoefficients of various species are not used due to the already largeuncertainties in estimates of other parameters. Combining theseassumptions, the estimated diffusion-limited current density for H₂oxidation is 2.5±1.5 mA cm⁻², which is fairly close to theexperimentally measured value (˜2.75 mA cm⁻², FIG. 10B).

Development of the gas-liquid interface across the SSC

FIGS. 17A-17G depict the development of the gas-liquid interface acrossthe vertical standing SSC and carbon-cloth (CC) based GDEs. As thefigures may not tell the complete picture, the process is described inwords below.

Initially, the gas compartment and electrolyte compartments contain noelectrolyte and are separated by a vertically standing SSC or CC GDE.When gas is fed to the gas compartment, it predominantly leaves throughthe GDE if any resistance to flow (by means of water column orotherwise) is applied to the outlet of the gas compartment. When a smallvolume of electrolyte (880 uL) is added, the SSC and CC GDEs behavedifferently.

In the case of the CC GDE, the electrolyte completely stays within theelectrolyte compartment and wets the GDE, and gas begins to flow outthrough the gas compartment outlet if there is not sufficient pressureto force the gas through the GDE. Adding additional electrolyte (for atotal of 1.75 mL) does not qualitatively change the picture.

In the case of a SSC GDE, the electrolyte actually goes through thevertically standing GDE into the gas compartment (FIGS. 17A-17G). Thegas usually escapes through the top of the SSC that has not been wet byelectrolyte if any resistance to flow is applied to the outlet of thegas compartment. When additional electrolyte is added (1.75 mL total),there is sufficient electrolyte between the two compartments to fullywet the SSC and prevent gas flow through the GDE until a pressuregradient with a magnitude greater than or equal to the magnitude of theLaplace pressure of the GDE is reached. The gas therefore temporarilyaccumulates in the gas compartment, and increases the absolute pressurein the gas compartment, evidenced by motion of the gas-liquid boundaryin the pressure controlling burette (FIGS. 32A-32D). This elevatedpressure actually pushes the electrolyte in the gas compartment throughthe SSC back into the electrolyte compartment. If the pressure gradientacross the SSC reaches the Laplace pressure of the SSC, gas begins toflow through the SSC (FIGS. 17A-17G and FIGS. 32A-32D). If it is lower,gas flows past the SSC out of the gas compartment (FIGS. 17A-17G andFIGS. 32A-32D).

Computing the diffusion-limited current density of N₂ reduction

The diffusion-limited current density for nitrogen reduction in a 1 MLiBF₄ in THF electrolyte has been estimated in Lazouski et al. Byfollowing a procedure similar to the one outlined above and in theaforementioned work, one can estimate the diffusion-limited currentdensity in an aqueous electrolyte at a flooded electrode by usingEquation 7.

$\begin{matrix}{J_{\lim} = \frac{6D_{N_{2}}C_{N_{2}}F}{\delta}} & {\#(7)}\end{matrix}$

The diffusion coefficient and solubility of nitrogen in pure water arewell known. At 25° C. and 1 bar of N₂ partial pressure, the diffusivityof N₂ in water is 2.01±0.1·10⁻⁹ m² s⁻¹, while the solubility is 0.66 mM.See, for example, Ferrell, R. T. & Himmelblau, D. M. Diffusioncoefficients of nitrogen and oxygen in water. J. Chem. Eng. Data 12,111-115 (1967); and Battino, R., Rettich, T. R. & Tominaga, T. TheSolubility of Nitrogen and Air in Liquids. J. Phys. Chem. Ref Data 13,563-600 (1984), each of which is incorporated by reference in itsentirety. The diffusion boundary layer thickness depends heavily on thehydrodynamics of the electrolyte; typical values for CO₂ reduction in anaqueous electrolyte are 60-160 μm; the boundary layer thickness may bethinner is well-defined and vigorous hydrodynamics are observed, such asin systems utilizing rotating disk electrodes (RDEs). See, for example,Weng, L. C., Bell, A. T. & Weber, A. Z. Modeling gas-diffusionelectrodes for CO₂ reduction. Phys. Chem. Chem. Phys. 20, 16973-16984(2018), which is incorporated by reference in its entirety. From thesevalues, it was found that the diffusion limited current density fornitrogen reduction in water is a mere 0.48-1.27 mA cm⁻².

Source of ammonia in two phases

It was found that produced NH₃ can be found in both the solution and thegas phases (FIGS. 12C and 12D). The rate at which NH₃ is produced in thegas phase is almost independent of the applied current density, whichimplies that the FE for NH₃ in the gas phase decreases with appliedcurrent density. It is unlikely that the NH₃ is formed directly in thegas phase, as both the lithium and proton source required for NH₃formation are in solution. The source of NH₃ in the gas phase may bestripping of the solution of NH₃ by the gas flowing past the SSC. Thiswould imply that NH₃ is much more concentrated at the electrode than inthe bulk. The general lack of forced mixing in the electrolyte supportsthis hypothesis. No ammonia is found in the gas phase when gas flowsthrough the electrode (FIG. 12D). Any gas phase ammonia would be dilutedin the bulk solution and thoroughly mixed, minimizing the amount ofstripping.

The lack of stripping of ammonia from the bulk solution is likely due tostrong interactions between Li⁺ and NH₃, stronger than between Li⁺ andTHF. See, for example, Kaufmann, E., Gose, J. & Schleyer, P. v. R.Thermodynamics of solvation of lithium compounds. A combined MNDO and abinitio study. Organometallics 8, 2577-2584 (1989), which is incorporatedby reference in its entirety. The formation Li—NH₃ complexes in solutionincreases the solubility of ammonia in the electrolyte (FIGS. 39A-39D),reducing the ammonia partial pressure at a given NH₃ concentration.

Estimating energy efficiency and consumption

A detailed description for estimating the energy efficiency of theprocess and sources of energy losses has been described in Lazouski etal. Briefly, the energy efficiency is computed as follows by the formulagiven in Equation 8.

$\begin{matrix}{\eta = \frac{U_{{NH}_{3}} \cdot {FE}}{V_{total}}} & {\#(8)}\end{matrix}$

In Equation 8, U_(NH) ₃ is the standard potential for the reaction ofammonia oxidation, i.e. the one likely to be used to extract useful work(4NH₃+3O₂→2N₂+6H₂O), V_(total) is the total applied potential across thecell, and FE is the Faradaic efficiency. In order to compute the energyconsumption, Equation 9 was used.

$\begin{matrix}{P = \frac{3{F \cdot V_{total}}}{{FE} \cdot M_{{NH}_{3}}}} & {\#(9)}\end{matrix}$

In Equation 9, F is Faraday's constant (96485 C/mol), V_(total) is thetotal applied cell voltage, FE is the ammonia Faradaic efficiency, andM_(NH) ₃ is the molar mass of ammonia (17 g/mol).

TABLE 1 Compilation of literature highest reported rates obtained innonaqueous electrolytes at close-to-ambient conditions. The data areplotted in FIG. 13A. Highest NH₃ NH₃ FE at Cathode production rate,highest Reference material Electrolyte Conditions nmol cm⁻² s⁻¹ rate, %(listed below) Ag—Au@ZIF 0.2M LiCF₃SO₃, ~1% Ambient 0.010 18.0 17 EtOH,THF Stainless steel [C₄mpyr][eFAP] Ambient 0.021 34.0 18 cloth, floodedCopper foil 1M LiBF₄, 0.2M Ambient 7.88 15.2 1 EtOH, THF Silver foil0.2M LiClO₄, 0.18M Ambient 0.58 8.4 19 EtOH, THF Iron foil 0.2M LiClO₄,0.18M 50 bar N₂, 3.99 57.7 EtOH, THF 25° C. Molybdenum 0.2M LiClO₄,0.18M Ambient 0.22 7.5 20 foil EtOH, THF Nickel foil 0.1M LiCl in EDAAmbient 0.036 17.2 21 Porous nickel 0.01M H₂SO₄in 9:1 Ambient 0.015 0.8522 2-propanol/water Polyaniline on 0.03M H₂SO₄, 0.1M 50 bar N₂, 0.44 1623 platinum LiClO₄, MeOH 25° C. Nickel LiClO₄, EtOH, THF Ambient 0.263.8 24 Nickel LiClO₄, THF, EtOH, Ambient 0.11 1.7 PYR-14 TFSI IonicLiquid Stainless steel 1M LIBF4, 0.1M Ambient 30.4 35.3 Present workcloth, GDE EtOH

TABLE 2 Raw data for current density variation for hydrogen oxidation onvarious substrates. The data are plotted in FIG. 12A. CurrentFerrocenium density, concentration, Ferrocenium HOR mA cm⁻²Electrode/condition mM FE, % FE, % 5 Pt/C, 0 kPa 5.87 78.21 21.79 5Pt/C, 0 kPa 7.39 98.57 1.43 10 Pt/C, 0 kPa 7.82 104.33 −4.33 10 Pt/C, 0kPa 6.27 83.56 16.44 15 Pt/C, 0 kPa 6.57 87.57 12.43 15 Pt/C, 0 kPa 6.3584.65 15.35 20 Pt/C, 0 kPa 6.87 91.67 8.33 20 Pt/C, 0 kPa 5.25 69.9930.01 25 Pt/C, 0 kPa 5.90 78.62 21.38 25 Pt/C, 0 kPa 7.48 99.82 0.18 5Pt/C 5 kPa 7.41 98.81 1.19 5 Pt/C 5 kPa 8.22 109.80 −9.80 10 Pt/C 5 kPa8.76 116.95 −16.95 10 Pt/C 5 kPa 7.14 95.32 4.68 15 Pt/C 5 kPa 6.8391.14 8.86 15 Pt/C 5 kPa 6.34 84.55 15.45 20 Pt/C 5 kPa 8.30 110.84−10.84 20 Pt/C 5 kPa 6.90 92.03 7.97 25 Pt/C 5 kPa 8.26 110.33 −10.33 25Pt/C 5 kPa 6.13 81.76 18.24 5 Pt/C 20 kPa 0.03 −0.05 100.05 5 Pt/C 20kPa 0.03 0.00 100.00 10 Pt/C 20 kPa 0.34 4.11 95.89 10 Pt/C 20 kPa 0.404.95 95.05 15 Pt/C 20 kPa 1.79 23.51 76.49 15 Pt/C 20 kPa 1.47 19.2580.75 20 Pt/C 20 kPa 3.90 51.80 48.20 20 Pt/C 20 kPa 3.15 41.83 58.17 25Pt/C 20 kPa 5.28 70.32 29.68 25 Pt/C 20 kPa 5.32 70.88 29.12 5 Pt/SCC, 1kPa 0.060 0.37 99.63 5 Pt/SCC, 1 kPa 0.047 0.20 99.80 10 Pt/SCC, 1 kPa0.040 0.10 99.90 10 Pt/SCC, 1 kPa 0.035 0.03 99.97 15 Pt/SCC, 1 kPa0.036 0.05 99.95 15 Pt/SCC, 1 kPa 0.038 0.08 99.92 20 Pt/SCC, 1 kPa0.081 0.65 99.35 20 Pt/SCC, 1 kPa 0.129 1.30 98.70 25 Pt/SCC, 1 kPa0.100 0.90 99.10 25 Pt/SCC, 1 kPa 0.043 0.14 99.86

TABLE 3 Raw data for pressure gradient variation for hydrogen oxidationon Pt/SSC. The data are plotted in FIG. 12B. Pressure Ferroceniumgradient, concentration, Ferrocenium HOR kPa mM FE, % FE, % 0 6.54 87.2812.72 0 7.24 96.63 3.37 0.5 0.08 0.60 99.40 0.5 0.34 4.14 95.86 1 0.02−0.12 100.12 1 0.08 0.65 99.35 1.5 0.07 0.49 99.51 1.5 0.10 0.90 99.101.9 0.05 0.21 99.79 1.9 0.11 1.01 98.99

TABLE 4 Raw data for current density variation for nitrogen reduction onsteel cloths. The data are plotted in FIG. 12C. Current NH₃ density,solution NH₃ gas NH₃ total NH₃ solution rate, NH₃ gas rate, NH₃ totalrate, mA cm⁻² FE, % FE, % FE, % nmol cm⁻² s⁻¹ nmol cm⁻² s⁻¹ nmol cm⁻²s⁻¹ 5 22.59 9.65 32.25 3.90 1.67 5.57 5 17.51 10.58 28.09 3.02 1.83 4.855 21.78 15.15 36.92 3.76 2.62 6.38 10 28.88 4.92 33.81 9.98 1.70 11.6810 37.66 9.94 47.60 13.01 3.43 16.44 10 35.80 9.01 44.81 12.37 3.1115.48 15 36.13 7.50 43.63 18.72 3.89 22.61 15 36.74 4.10 40.84 19.032.12 21.16 15 31.84 5.74 37.58 16.50 2.98 19.47 20 30.76 5.38 36.1421.25 3.71 24.97 20 40.23 4.85 45.07 27.79 3.35 31.14 20 29.46 3.2732.74 20.35 2.26 22.62 20 33.47 8.14 41.62 23.13 5.62 28.75 25 36.095.52 41.61 31.17 4.76 35.93 25 33.43 4.88 38.31 28.87 4.22 33.09 2526.70 2.25 28.94 23.05 1.94 24.99 25 30.01 2.14 32.15 25.92 1.85 27.77

TABLE 5 Raw data for pressure gradient variation across the electrodefor nitrogen reduction on steel cloths. The data are plotted in FIG.12D. Pressure gradient, NH₃ solution NH₃ gas Total NH₃ solution rate,NH₃ gas rate, NH₃ total rate, kPa FE, % FE, % FE, % nmol cm⁻² s⁻¹ nmolcm⁻² s⁻¹ nmol cm⁻² s⁻¹ 0 2.32 — 2.32 1.20 — 1.20 0 4.22 — 4.22 2.19 —2.19 0.5 44.00 6.20 50.20 22.80 3.21 26.01 0.5 41.72 3.12 44.84 21.621.62 23.24 1.0 20.67 6.74 27.41 10.71 3.49 14.20 1.0 36.13 7.50 43.6318.72 3.89 22.61 1.0 36.74 4.10 40.84 19.03 2.12 21.16 1.5 31.84 5.7437.58 16.50 2.98 19.47 1.5 32.36 5.85 38.21 16.77 3.03 19.80 1.7 40.113.01 43.12 20.78 1.56 22.34 1.7 42.15 0.09 42.25 27.93 0.08 28.01

References, each of which is incorporated by reference in its entirety.

-   1. Lazouski, N., Schiffer, Z. J., Williams, K. & Manthiram, K.    Understanding Continuous Lithium-Mediated Electrochemical Nitrogen    Reduction. Joule 3, 1127-1139 (2019).-   2. Nielander, A. C. et al. A Versatile Method for Ammonia Detection    in a Range of Relevant Electrolytes via Direct Nuclear Magnetic    Resonance Techniques. ACS Catal. 9, 5797-5802 (2019).-   3. Stoychev, D., Papoutsis, A., Kelaidopoulou, A., Kokkinidis, G. &    Milchev, A. Electrodeposition of platinum on metallic and    nonmetallic substrates—selection of experimental conditions. Mater.    Chem. Phys. 72, 360-365 (2001).-   4. Rao, C. R. K. & Trivedi, D. C. Chemical and electrochemical    depositions of platinum group metals and their applications. Coord.    Chem. Rev. 249, 613-631 (2005).-   5. Baumgartner & Raub. The Electrodeposition of Platinum and    Platinum Alloys. Platin. Met. Rev. 32, 188-197 (1988).-   6. Gagne, R. R., Koval, C. A. & Lisensky, G. C. Ferrocene as an    Internal Standard for Electrochemical Measurements. Inorg. Chem. 19,    2854-2855 (1980).-   7. Singh, A., Chowdhury, D. R. & Paul, A. A kinetic study of    ferrocenium cation decomposition utilizing an integrated    electrochemical methodology composed of cyclic voltammetry and    amperometry. Analyst 139, 5747-5754 (2014).-   8. Bard, A. J. & Faulkner, L. R. Electrochemical Methods.    Fundamentals and Applications. (John Wiley & Sons, Inc, 2001).-   9. Gibanel, F., López, M. C., Royo, F. M., Santafé, J. &    Urieta, J. S. Solubility of nonpolar gases in tetrahydrofuran at 0    to 30° C. and 101.33 kPa partial pressure of gas. J. Solution Chem.    22, 211-217 (1993).-   10. Brunner, E. Solubility of Hydrogen in 10 Organic Solvents at    298.15, 323.15, and 373.15 K. J. Chem. Eng. Data 30, 269-273 (1985).-   11. Weisenberger, S. & Schumpe, A. Estimation of Gas Solubilities in    Salt Solutions at Temperatures from 273 K to 363 K. AIChE J. 42,    298-300 (1996).-   12. Ferrell, R. T. & Himmelblau, D. M. Diffusion coefficients of    nitrogen and oxygen in water. J. Chem. Eng. Data 12, 111-115 (1967).-   13. Battino, R., Rettich, T. R. & Tominaga, T. The Solubility of    Nitrogen and Air in Liquids. J. Phys. Chem. Ref Data 13, 563-600    (1984).-   14. Weng, L. C., Bell, A. T. & Weber, A. Z. Modeling gas-diffusion    electrodes for CO₂ reduction. Phys. Chem. Chem. Phys. 20,    16973-16984 (2018).-   15. Kaufmann, E., Gose, J. & Schleyer, P. v. R. Thermodynamics of    solvation of lithium compounds. A combined MNDO and ab initio study.    Organometallics 8, 2577-2584 (1989).-   16. Lobaccaro, P. et al. Effects of temperature and gas-liquid mass    transfer on the operation of small electrochemical cells for the    quantitative evaluation of CO₂ reduction electrocatalysts. Phys.    Chem. Chem. Phys. 18, 26777-26785 (2016).

17. Lee, H. K. et al. Favoring the unfavored: Selective electrochemicalnitrogen fixation using a reticular chemistry approach. Sci. Adv. 4,eaar3208 (2018).

-   18. Zhou, F. et al. Electro-synthesis of ammonia from nitrogen at    ambient temperature and pressure in ionic liquids. Energy Environ.    Sci. 10, 2516-2520 (2017).-   19. Tsuneto, A., Kudo, A. & Sakata, T. Lithium-mediated    electrochemical reduction of high pressure N₂ to NH₃ . J.    Electroanal. Chem. 367, 183-188 (1994).-   20. Andersen, S. Z. et al. A rigorous electrochemical ammonia    synthesis protocol with quantitative isotope measurements. Nature    570, 504-508 (2019).-   21. Kim, K., Yoo, C., Kim, J., Yoon, H. C. & Han, J. Electrochemical    Synthesis of Ammonia from Water and Nitrogen in Ethylenediamine    under Ambient Temperature and Pressure. J. Electrochem. Soc. 163,    F1523-F1526 (2016).-   22. Kim, K. et al. Communication—Electrochemical Reduction of    Nitrogen to Ammonia in 2-Propanol under Ambient Temperature and    Pressure. J. Electrochem. Soc. 163, F610-F612 (2016).-   23. Köleli, F. & Röpke, T. Electrochemical hydrogenation of    dinitrogen to ammonia on a polyaniline electrode. Appl. Catal. B    Environ. 62, 306-310 (2006).-   24. Pappenfus, T. M., Lee, K., Thoma, L. M. & Dukart, C. R. Wind to    Ammonia: Electrochemical Processes in Room Temperature Ionic    Liquids. in ECS Transactions vol. 16 89-93 (ECS, 2009).

Another example of a standalone electrode architecture is depicted inFIG. 42A. A version of the drawing with see-through edges is shown inFIG. 42B. The pieces, left to right, are: gas compartment 100,compartment O-ring 102, current collector 103, GDE material (mesh) 104,holder O-ring 106, and holder 108. These components form an enclosed gaselectrode as describe herein. Ports 110 provide gas access to theelectrode surfaces, allowing exposure to reactants and removal ofproducts.

Referring to FIG. 43, a flow in electrode configuration can include apower connection 120 can enter the electrode to access the electrodesurface within a flow-in electrode 140. A gas inlet 150 provides a fluidconnection of gas through port 110. A pressure regulator 160 can providecontrol of pressure within the standalone electrode. For example, thepressure regulator can be a water column or other fluid column.

Referring to FIG. 44, a flow past electrode configuration can include apower connection 120 can enter the electrode to access the electrodesurface within a flow-past electrode 190. A gas inlet 150 provides afluid connection of gas through a port 110. A pressure regulator 160 canprovide control of pressure within the standalone electrode. Forexample, the pressure regulator can be a water column or other fluidcolumn. Gas outlet 180 can exit a port 110 and through the pressureregulator 160.

Referring to FIG. 45, an exemplary electrochemical reaction systemutilizing a flow-in standalone GDE electrode and a flow-past standaloneGDE electrode is shown. The system 200 can include a flow-in standaloneGDE electrode 140 and a flow-past standalone GDE electrode 190, whichare immersed in an electrolyte 220. Flow-in electrode 140 can transforma gas flow to a reactant gas that can then be used in conversion of areactant to a product in the flow-past electrode 190.

An example is further described here.

Stand-alone electrode for utilizing sparingly soluble gases

Metallic meshes and other porous materials as gas diffusion electrodescan generally utilize sparingly soluble gases in electrochemicalreactions in aqueous and nonaqueous electrolytes. However, in someapplications, the gas diffusion electrodes are utilized in a custom,parallel-electrode architecture. While the architecture is efficient andconvenient for both testing and synthetic applications, there may beapplications that require utilization of sparingly soluble gases inother architectures. Examples include rapid resting of reactions ineasy-to-setup beaker cells, synthetic reactions which require largevolumes of solvent, and those which use gases in counter and balancingreactions. For these and other applications, a stand-alone electrode isdescribed herein that uses the nonaqueous GDEs for utilizing sparinglysoluble gases in electrochemical reactions.

The basic standalone architecture can be seen in FIGS. 42A and 42B. Thearchitecture consists of a gas compartment, a current collector, theactive gas diffusion electrode, and a holder piece, with O-rings betweenrelevant parts of the setup. The gas compartment is meant to provide aseparate reactive gaseous phase at the electrode. The current collectoris immediately in contact with the gas compartment via an O-ring. Thegas compartment contains an electrical connection—a wire of copper,aluminum, iron, nickel, or other metal, permanently or non-permanentlyfixed to the inside walls of the gas compartment. The current collectorgoes around the entire edge of the gas compartment opening and isslightly wider at the outer edge than the O-ring to maintain a goodseal. The GDE is then in immediately contact with the current collector.The inner edge of the current collector is smaller than the GDE to haveuniform electrical contact with the GDE around the edge. The connectorhas a groove to hold another O-ring which surrounds the GDE. The GDE issupported by a lip on the connector piece—the construction iseffectively gas-tight due to O-ring contacting, and allows gas onlythrough the electrode. The assembly can be held together with bolts madeof metal or plastic, preferably plastic so as to avoid contamination ofmetals in the electrolyte. Other bolt-less configurations, some wherethe second, holder piece of plastic is screwed on using threading, arepossible.

There are a number of possible configurations of the standaloneelectrode. In particular, the electrode may be in a “flow in”configuration, where the gas is fed through one inlet and used up by thereactions occurring at the electrode (FIG. 43), in a “flow past”configuration, where the gas enters the gas compartment through an inletand exits as either unreacted gas or gaseous products through a separateoutlet (FIG. 44), or a “flow through” configuration, where the gas isforced through the GDE into the electrolyte solution. In all theseconfigurations, the electrical contact may enter the gas compartmenteither through one of the gas inlets, if chemical compatibility allows,or through a separate, gas- and liquid-tight inlets, e.g. via a septum.In FIGS. 43 and 44, the electrical contact is depicted to enter throughthe gas inlet.

One of the key requirements for invention operation is controlling thepressure gradient across the GDE. The location and configuration of thepressure control differ somewhat in the three gas flow configurations.In the “flow-through” configuration, the GDE itself establishes thenecessary pressure gradients, and simply flowing gas into the gascompartment at a high enough rate and/or positive pressure issufficient.

In the “flow-past” configuration, the pressure in the gas compartment iscontrolled after the gas leaves standalone electrode (FIG. 44). In oneimplementation, a simple water column is sufficient to control thepressure inside the gas compartment. Typically, the gas is continuouslyflowed into and out of the gas compartment into a water column, whichadds additional positive pressure to the gas. More advanced pressureregulation methods may be used, such as a backpressure regulator.Further operations with the outlet can be performed after pressurecontrol, assuming they do not add additional back-pressure to the gas.

In the “flow-in” configuration, the pressure in the gas compartment iscontrolled prior to entering electrode (FIG. 43). In this case, the gasstream is branched, with one stream entering the electrode, while theother is used to control the gas pressure of all the gas in the system.A static water column or more advanced pressure regulators can be usedto maintain a constant pressure of gas. If the gas flow into theelectrode is equal to its rate of utilization in the electrochemicalreaction (and any leaks), then gas does not have to leave via thepressure-controlling system. However, if excess gas enters the system,then the excess gas can be expelled through the pressure regulator, asto avoid “flow-through” behavior at the GDE. In this case, the GDE willutilize a fraction of the gas fed to the system by pulling it into theelectrode compartment.

Demonstration of standalone electrode use

A flow-in standalone electrode with a platinum-coated stainless-steelcloth GDE was assembled to demonstrate the capability of the electrodeto utilize gases in electrochemical reactions in various solvents. Thestandalone electrode was used as the anode, to which either nitrogen orhydrogen gas was fed. A platinum foil was used as the cathode and anAg/AgCl electrode was used as a reference. Two electrolyte compositionswere used sequentially with the same electrode to demonstrate thesolvent-agnostic nature of the electrode: 0.1 M tetrabutylammoniumtetrafluoroborate with 0.05 M hexafluoroisopropyl alcohol inacetonitrile was used as a nonaqueous electrolyte, while a 0.05 M Na₂SO₄in water solution was used as an aqueous electrolyte. Either nitrogen orhydrogen gas was first flowed into the electrode in a flow-throughconfiguration to fill the gas compartment of the electrode and saturatethe electrolyte solution, after which the pressure in the electrode gascompartment was decreased to put the electrode in a flow-inconfiguration. A linear-sweep voltammogram was measured by sweeping thepotential from open circuit voltage to a high oxidation potential, firstwith nitrogen in the gas compartment, followed by having hydrogen in thecompartment. A significantly higher current was obtained when usinghydrogen as the feed gas when compared to using nitrogen, whichdemonstrates that hydrogen oxidation is occurring. The currents obtainedsignificantly exceeded the diffusion limited hydrogen oxidation current,demonstrating the use of the standalone electrode as a gas diffusionelectrode. Results of the exemplary reaction are shown in FIGS. 46A-46B.

Details of one or more embodiments are set forth in the accompanyingdrawings and description. Other features, objects, and advantages willbe apparent from the description, drawings, and claims. Although anumber of embodiments of the invention have been described, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. It should also be understood thatthe appended drawings are not necessarily to scale, presenting asomewhat simplified representation of various features and basicprinciples of the invention.

What is claimed is:
 1. An electrochemical system comprising: a housingincluding a chamber; an electrode within the housing; and a gaspermeable metal on a surface of the electrode in contact with thechamber.
 2. The system of claim 1, further comprising a gas inlet to thehousing.
 3. The system of claim 1, further comprising a first outlet ofthe housing to release a product from the housing.
 4. The system ofclaim 1, wherein the gas permeable metal includes a metal mesh.
 5. Thesystem of claim 4, wherein the metal mesh includes 100, 200, 300, 400,500, 1000, 1500, or 2000 fibers per inch.
 6. The system of claim 1,wherein the gas permeable metal includes openings of between 1 and 200micrometers, preferably between 2 and 100 micrometers.
 7. The system ofclaim 1, wherein the gas permeable metal includes metal fibers or aporous metal.
 8. The system of claim 1, wherein the gas permeable metalincludes stainless steel, steel, nickel, iron, copper, silver, gold, orplatinum.
 9. The system of claim 1, wherein the gas permeable metalincludes a catalytic metal, metal oxide, metal sulfide, or metalphosphide.
 10. The system of claim 1, wherein gas permeable metal isexposed to a pressure gradient.
 11. A method of supplying a gas to anelectrochemical system comprising: contacting a gas with a gas permeablemetal on a surface of an electrode in a chamber of a housing.
 12. Themethod of claim 11, wherein the gas is a sparingly soluble gas.
 13. Themethod of claim 11, further comprising supplying a pressure of the gasin the chamber to create a pressure differential at the electrode. 14.The method of claim 11, further comprising applying a voltage to theelectrode.
 15. The method of claim 11, wherein the gas permeable metalincludes a metal mesh.
 16. The method of claim 15, wherein the metalmesh includes 100, 200, 300, 400, 500, 1000, 1500, or 2000 fibers perinch.
 17. The method of claim 11, wherein the gas permeable metalincludes openings of between 1 and 200 micrometers, preferably between 2and 100 micrometers.
 18. The method of claim 11, wherein the gaspermeable metal includes metal fibers or a porous metal.
 19. The methodof claim 11, wherein the gas permeable metal includes stainless steel,steel, nickel, iron, copper, silver, gold, or platinum.
 20. A method ofoxidizing or reducing a gas comprising: contacting a gas with a gaspermeable metal on a surface of an electrode.
 21. The method of claim20, wherein the gas is a sparingly soluble gas.
 22. The method of claim20, wherein the gas is hydrogen.
 23. The method of claim 20, wherein thegas is nitrogen.
 24. The method of claim 23, wherein the ammonia isproduced at a Faradaic yield of at least 30% or at least 40%.
 25. Themethod of claim 20, wherein supplying a pressure of the gas in thechamber to create a pressure differential at the electrode.
 26. Anelectrochemical system comprising: a first electrode including: ahousing including a chamber; an electrode within the housing; and a gaspermeable metal on a surface of the electrode in contact with thechamber; and a second electrode including a gas inlet to a housingincluding a gas permeable metal on a surface of an electrode and a firstoutlet to release a product from the system.
 27. The system of claim 26,wherein each gas permeable metal includes a metal mesh.
 28. The systemof claim 27, wherein each metal mesh includes 100, 200, 300, 400, 500,1000, 1500, or 2000 fibers per inch.
 29. The system of claim 26, whereinat least one gas permeable metal includes openings of between 1 and 200micrometers, preferably between 2 and 100 micrometers.
 30. The system ofclaim 26, wherein at least one gas permeable metal includes metal fibersor a porous metal.
 31. The system of claim 26, wherein each gaspermeable metal is exposed to a pressure gradient.